Running like crap!

Had a similar situation with my Swinger. Been running great all summer, now it suddenly runs like crap, but seemed to smooth out when you give it some RPMs. Threw plugs in it, even though the old ones looked okay. Checked the distributor for frozen advance weights/busted springs, cap and rotor looked normal. All good.
Started thinking timing chain, but checked and confirmed all was as it should be. I even ran fuel from a gas can into it, in case I got a tank of bad gas... no dice.
Then I noticed a bit of fog in the engine compartment, and a little water dripping from the right side exhaust. Opened the cool radiator cap, and the darned thing was spurting up like Old Faithful. Put the compression tester on it and sure enough, 4 & 6 were down. Head gasket time (if I'm lucky).
Like you said, end of season anyway, so put it away for the winter. When I get time I'll probably just yank it and freshen the whole thing. Like I didn't have anything else to do...
Hope you fare better.