Ok, I've found the problem, but I don't know where it's coming from. On the chassis/backplane of the projector, there are 2 connectors that go to the tube socket. One goes to the driver board and then to the tube socket, the other goes directly to the socket. On the one that goes directly to the tube socket, there is a black wire and 4 yellow wires. The black wire has a high voltage (>700V). when the tubes go into "pink Floyd" mode, that voltage drops out/cuts out/etc.. When it's stable, it's a nice DC flat line. So I'm guessing there is something that causes the voltage to drop out, either because the supply is flaking out or there is a short somewhere that is pulling it down.
Ok, since my manual is for a 808s and not all parts are interchangeable,, is it relatively safe to assume that the backplane connections are the same and I can focus in on the G2 driver board? I can try to pull the connector for each tube socket to see if there is an intermittent short that pulls it down, otherwise I'm onto the G2 supply.
[update] I swapped out the 762120S that converts 15kv down (it's the potted brown block that's screwed down to the G2 board. that's not it either. I also pulled the connectors to each gun (one at a time), and that also didn't fix it.
There is always too much month left at the end of the money.