base timing keeps advancing by itself...
@Phreakish that would not explain why he has to back up the distributor further than it was previously.
That depends. I don't think we have an exactly clear picture of what's happening. I would expect to have to back it up 'further' in order to re-obtain the set idle speed until the engine cools down. If it keeps moving, at some point the vacuum canister is going to hit something. It's unclear if that's happened. If it's moving 10-20 degrees each time, then he'd be out of space to turn the distributor after 1-2 times of this happening, but it sounds like it's happened more than that.
I suspect what is happening is the following:
Engine cold, idle timing at 12. Rpm sub 700.
Engine warm, idle timing at 12. Rpm sub 800.
Engine hot, timing now reads 22. Rpm 1600-2200 (hanging).
Engine hot, idle timing at 8. Rpm sub 900.
Engine cold, idle timing 8. Rpm 6-700.
Engine warm, idle timing 8. Rpm sub 800 - diminished performance.
Engine hot, timing now reads 18. Rpm 1200-1600 (hanging, but lower).
Engine warm (from hot to warm while adjusting timing), timing now reads 12. Rpm sub 900.
Engine hot, timing reads 22 again. Rpm 1600-2200.
etc...
During the time it takes to reset the timing lower to get the rpm back to idle, the engine loses enough heat that it would probably idle normal again, but now it has less advance too. The fixation on the timing leads to ignoring other variables. I would get it hot and get it to 'advance' and the high idle. Then manually adjust the idle speed down (write down how much the idle screw is turned) then once back to a normal idle speed, re-check the timing. It should be back to normal - if not, THEN we know it's an issue. Timing and rpm vary together so any two timing values at different RPMs do not correlate to one another.
What we do know is that it's advancing. That rules out anything slipping, since it would result in retarding the timing.
If the distributor was not being adjusted back and forth, the distributor would be getting rotated round-and-round. No mention so far of something like that (having to re-order plug wires on the cap, re-stab the distributor, etc).
So, with the information as presented, and the new info that it goes 'back' (to normal? To something else?) when cold, it's either an electronics issue or my theory as presented above.
I find it tough to fully explain through text alone, so it may not be totally clear what I'm trying to state. It would be pretty easy to see in-person though, and I've experienced it.