Why do McIntosh SS amps cut out?

That's usually a relay going bad. The way to test for sure
is next time a channel cuts, instead of cycling the power
off/on, turn off the speakers and turn the volume up about
3/4 for about 5 - 10 seconds. Turn the volume back down to
normal and turn the speakers back on. If both channels are
back it's definitely a relay. Sometimes cleaning can help
sometimes they're shot beyond hope. I'm sure Mac has the
replacements if necessary.

Faulty relays is an extremely common problem with vintage
SS amps of every make and model. Paradoxically, I was told
by a tech I trust that many relays go bad because the amps
are never driven above a few milliwatts on average. The
current has a self-cleaning effect on the relay contacts so
relays actually benefit from a good workout once in a while.

3 out of 4 of my vinatge receivers have relay issues like
you describe. Interestingly, only my McIntosh MAC-4100
receiver (1979) still has reliable relays.