Intel iMac Ethernet intermittently not working

Then I would suggest that you run an EtreCheck report, and optionally, post it here if you need assistance with interpreting its results. If you are not familiar with EtreCheck, it is a diagnostics tool, developed by a valued forum member, that greatly aides in troubleshooting difficult Mac issues ... software or hardware.

Ref: Using EtreCheck to Troubleshoot Potential… - Apple Community

You've installed what most of the experienced users here consider to be akin to malware: CleanMyMac.

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Uninstall CleanMyMac according to the developer's instructions. You can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For CMM software you'd do the following search(es): 

1 - Name contains cleanmymac

2 - Name contains macpaw

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

If you get warnings that the file can't be deleted because it is in use or used by another app boot into Safe Mode according to

and delete from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Doing a quick review of the report, I have the following observations/recommendations:

• Re-enable your Mac's security features. It appears that you have both the Mac's Automatic & Security Updates disabled.
• Completely remove CleanMyMac and any other "cleaner" apps from your Mac. They serve no useful purpose and have been known to cause more issues than resolve them.
• Since your Mac has a Fusion drive, I strongly suggest that you consider testing the vitality of this drive set with a diagnostic tool, like DriveDx. The HDD side of these Fusion drives tend to fail and can be a cause of poor performance.
• Your Mac as an enormous (I would say more like an insane) amount of Audio Plug-ins. Do you really need all of them?
• Google Chrome is known to be a "memory hog." You may want to consider using another browser. If you don't like Safari, but prefer the Chrome-like interface, I suggest trying the Brave browser.
• Take the advice under the Clean Up section to remove the items listed there. They are associated to apps that are no longer on your Mac.
• Under the Diagnostics Information section a number of processes (signpost_reporter) and apps (Logic Pro X & iZotope RX) that were reported as using a significant amount of CPU processing within the last 60 days. A bit more significant is the Apple mds_stores process crashing 4x during that same time period. mds_stores is related to the Apple Spotlight. This can also relate to the Time Machine backup process.You may want to consider rebuilding the Spotlight's Index.

Okay 2 major items that I see.

CleanMyMac is still installed. You need to download and run FindAnyFile and have it search for anything that contains both:

• CleanMyMac
• MacPaw

move the files to the Trash and empty the Trash.

The second issue is CMM disabled Apple’s security and your security Mac is now at risk of malware. It has also likely damaged Mac OS. After you have totally gotten rid of CMM the you need to boot into the Recovery Partition and install Mac OS to fill in any missing or damaged parts of Mac OS.

BTW, do you have any of the following third party apps installed on your Mac?

·      Antivirus Apps

·      Cleaning Apps

·      Security Apps

·      VPN Apps

·      Maintenance Apps

Please remember to post your EtreCheck report, the more eyes that view it the more it reveals.

remnant_simplyreal wrote:

• I'm not familiar with what a Fusion drive is, but I have not had any hard drive issues so far. I can run DriveDx, but does that have any relation to the Ethernet issue?

Fusion is a big cheap and slow hard disk drive (HDD) with a dinky and fairly speedy SSD as a performance cache.

Fusion uses logic (that tries) to shuffle the most active storage from the HDD onto the SSD, and shuffle the least active of the storage from the SSD and onto the HDD.

Fusion an elegant kludge that tries to get closer to SSD performance from a cheaper and more voluminous HDD. When it works, it’s a somewhat-faster HDD, at not-SSD prices.

When Fusion is split, or when either part of Fusion fails and reports errors, Fusion can misbehave in various ways. Pretty much all of those involve some degree of performance degradation, and some involve data loss.

If your main storage is malfunctioning, Ethernet might run at full speed, but reads or writes to storage might not. Which means various ancillary app and system functions can run much more slowly.

Unbelievable all that junk,on that Imac!

CleanMyMac is bad, but Pace Anti-Piracy Software is not better at all!

I am not an expert,on the, issue with your iMac!

A reinstall from the Time Machine is not an option,i think?

Performance:

System Load: 1.22 (1 min ago) 1.57 (5 min ago) 1.77 (15 min ago)

Nominal I/O usage: 0.17 MB/s

File system: 16.82 seconds

Write speed: 744 MB/s

Read speed: 2621 MB/s

Those speeds are in range for a healthy Fusion drive in that model iMac. The Writes could be a bit low but may increase when you remove CleanMyMac. I've seen plenty of Etrecheck reports here where removing CMM improved drive scores.

Macs, catlike, clean themselves and have for over 20 years. The industry knows that so, IMHO, any product that today claims to "clean" a Mac is....well, I'll stop there before the Profanity Police eject me.

Also rerun the test with all your external drives disconnected. I've recently worked several posts here where an older USB external reduced overall performance.