Music cutting out- not clipping but weird...

I'm looking for a place to start, although I've tried a lot of things. I am vinyl only, with table and SUT going into a Lamm LP2.1 to a LL2.1 and then to a Sun Audio 300B SET amp. I have a problem where once time per 2-3 hour listening session the music cuts out of the left channel and then the right channel fades out and then it all just pops back on. This never lasts more than a 3-4 seconds. It never happens more than once in a listening session. Nothing is clipping- it's not dependent on the sound level. It can't be power- nothing is powering down. None of the indicator lights is flickering and no protection circuits are cutting in. It's not the turntable/cartridge/SUT or cable because those have been completely swapped out. I've tried jiggling the cables from the phono pre to the pre and the cable from the pre to the amp while it's happening and nothing changes it. So it seems internal... right? Any suggestions on where to start looking?