Final Cut Pro timeline is suddenly glitchy on playback, seems corrupted

Booting off an external SSD with a fresh OS install allows you to confirm whether or not there are issues with the macOS installed on your iMac's internal drive. For this simple external SSD test just create a new user in the new OS and go from there. Install FCP, create a new library and import fresh media onto the external SSD.

Migration means using Apple's Migration Assistant to copy a System's info and Users from another computer, Time Machine backup or another backup drive. This is great when you buy a new machine and want to migrate yourself from your old machine to the new machine. The caveat is that it's best to only migrate User info, no Applications or System files.

Correct, just to rule things out, no disk encryption (FileVault) to make the test as straightforward as possible.

Ultimately the goal is to quickly take the iMac's currently installed macOS out of the equation to see if it's the problem.

If there was a bad drive in the Jellyfish that could also cause glitchy playback and random freezes.

+1 on Etrecheck. It is reliable and often the way we can find the causes of problems.

Before we wait for it, a few things come to mind:

Disk space?

You could try deleting all render files. They use a lot of space and a "stuck" render file could cause playback glitches.

One thing you could try is to delete the preferences.

Restart your Mac whilst holding down the Option and Command keys. In the window that opens click on the Delete preferences button.

FCP will open with an empty Library so you will have to navigate to the Library you were working on and double-click it so that it opens in FCP.

On further reflection, your problem sounds more like the classic symptoms of corruption in your project as you first suspected.

There is one simple (almost) foolproof test for this . . . close your existing Library/Project and open a new Library, import some new random video clips, create a new project in which you add effects, transitions, colour correction and see how it goes. Don't use the original clips as one of them may be corrupt and you are back where you started.

If everything is fine, it would indicate your original Library has a corrupt clip etc.

The whole test need take no longer than 5 minutes as you are not editing but just throwing things randomly into the timeline to create stress.

EDIT: You mentioned that it started after importing iPhone clips. Get rid of them and see if matters improve.

Even if they are the cause, deleting them may not get rid of the glitches as they may have contaminated the project in some way but it's worth trying.

Please post a fresh Etrecheck report.

Also, try creating a new admin user, login as that user, create a new FCP library and add a handful of clips representative of what you typically work with and see if the behavior changes.

Congrats on making progress Matthew!

Easy enough to reinstall FCP on the iMac drive and see if that helps. I would delete FCP preferences (hold command and option when launching FCP) and also delete all render files in the problem library.

I'm told that reinstalling the OS on recent macOS versions is a lot easier than before. Maybe Luis or Tom can chime in with a link to instructions. If you do go that route, make a couple backups first and then when you migrate back to the fresh OS install, *only* migrate your User info, no System Files, no Applications and no Network files. Download fresh apps from the AppStore or 3rd party sites.

**EDIT**

I just looked through your etrecheck report again and have a question about Zerotier. What do you use this for and is it active while you're editing?

FileVault can in fact degrade FCPX and other higher end app performance. It should never be used for video editing or 3D graphics. And it only protects from hackers trying to hack a hard drive. If someone steals your laptop and gets logged in, it won't matter, they still have access to your data. It's not all that serious of a security measure, but required by some industries that Apple sells to. Always leave it turned off.

Here are simple instructions to install a clean system in a fairly recent mac (any Apple Silicon, and Intel from around 2019 or perhaps 2018, depending on model).

1) Full backup using Time Machine; if possible, a second backup 2) System Settings->General->Transfer or Reset->Erase All Content and Settings This will leave only the OS itself, and erase all system modifications as well as all accounts, so you did do the backup beforehand, right? 3) When asked, choose to migrate from "another mac, drive or Time Machine backup"; select the TM backup drive used in step 1 4) Select to migrate ONLY the user accounts (no applications, settings or other files - otherwise you'd just be back to where you started) 5) Enjoy a properly working mac. Gradually install the applications that you do need. Never install any "cleaners" or "antivirus".

You have Chrome and its evil keystone daemons or agents. They wreck havoc on video applications.

See https://chromeisbad.com for an explanation of what they do and how to remove them.

Please post an EtreCheck report https://etrecheck.com giving the application Full Disk Access in Privacy & Security System Settings. Copy the report and use the Add Text button to paste the result into reply box. 

It's not Google's online services, it's the Google Chrome browser installation on your drive. It installs ugly, nasty, proprietary video deamons that don't play well with FCPX. Been an issue for awhile. Uninstalling Chrome browser has fixed many issues.

Etrecheck is a diagnostic utility by a respected member of this community.

It is used many times here every day to let those who are trying to assist know about hardware configurations and possible software issues in the affected user's Mac.