ethernet disconnects

If what you crave is stability over speed, you can arm-wave all of this by manually setting the Ethernet speed to Gigabit Ethernet and be done.

You can also increase the MTU (frame size) for modestly better performance), Up from default 1500 to about 8,000 works fine, higher requires some fiddling.

Have you contacted Apple support to make a trouble ticket and formally identify the problem?

Does this happen when you boot to Safe Mode?

That boot up takes minutes and clears caches and other things that may resolve your problem.

How to use safe mode on your Mac - Apple Support

I have not seen your specific problem reported in this forum before. However, there has been tro other problems with an ethernet connection.

MAC STUDIO M1 MAX: Ethernet port not working

The Mac Studio has a 10GB Ethernet port. If you have some fancy equipment at the other end of the cable, it is possible it is trying to make a 10GB connection.

A 10GB (or 5GB or 2.5GB) connection is only stable when cables are excellent and fairly short (like Category-6 rated cables under 100 feet). If either of those are not true, or you have you added patch cables that are not Category-6 rated, you could be seeing it connect at a faster-than-Gigabit speed, then erroring out and disconnecting.

The good way to check the actual connection speed USED to be Network Utility, But in Big Sur and later, Apple has deprecated network Utility and now you have to use a Terminal command to see your actual connection speed. First, you need to know what en number the link is. then you use a command like this one, susbstituting the actual en number.

My added 10GB Ethernet card is en5, so I use this:

 ifconfig en5 | grep media

with this as my output:

	media: autoselect (10Gbase-T <full-duplex,flow-control>)

to see if an Ethernet link is throwing more than a handful of initial errors, you can use Terminal command:

my main connect uses en5 (as shown in system preferences > network:

netstat -I en5

This is the resulting output. Counters are In-packets, In-errors, Out-packets, out-Errors. There should never be more than handful of errors from starting up, and in most cases, NONE.

Name       Mtu   Network       Address            Ipkts Ierrs    Opkts Oerrs  Coll

en5   8163  <Link#4>    00:01:d2:1a:00:dd   696697     0   484301     0     0

en5   8163  grantsmacpr fe80:4::461:ea0d:   696697     -   484301     -     -

en5   8163  192.168.0/23  192.168.0.204     696697     -   484301     -     -

If the link were running beyond its ability to run stably, for example it auto-speeded to 10GB but the cabling could only reliably support 2.5GB, we woud see non-zero errors counts, and errors increasing over time. (and possibly, disconnecting)