I find this kind of intermittent and short duration problems to be some of the most frustrating to diagnose. It sound like it is somewhat repeatable, though, which is better than something completely random, and 2 seconds is not impossibly short. I usually try to figure out if the problem is in the preamp or the power amp. You have an effects loop on this amp, so you could send the "preamp out" to some other amp or mixing board. When the FM212r cuts out you can see if the sound is still coming out of other device. If so that means the preamp is OK and the problem is "after" that. If everything cuts out at once the problem is in the preamp. Similarly you can take a line level from some other amp or mixer and put it into the "power amp in" on the FM212r and play it for 30+ minutes and see if it cuts out at all to test the power amp section by itself.
Once you get beyond that though it will likely mean having to open up the amp. Is this something that you are planning on trying to fix yourself? Since you sent it in for service before I am just trying to gauge your level of experience and comfort working on electronics.
Schematic in post #4 of this thread (https://music-electronics-forum.com/...ad.php?t=15209).