Before you dig into the carb, are you positive you don't have a vacuum leak anywhere?.
Besides checking lines, fittings, advance system components plus power brake and modulator parts if applicable, I seem to recall some Oldsies had a ported point on top of the intake that required a sealed bolt intstalled if unused?.
Always tough replicating problems in the shop that only show up driving under load at certain revs but if a manual inspection of the components show nothing amiss, sometimes intake leaks can be found at stand-still by hooking up a remote tach and trying different constant revs while keeping a close eye on the tach for variations as you mist methal-hydrate out of a spritzer bottle onto suspect vacuum areas...be careful of course and keep a fire extinguisher at hand!.
May be best if equipped to isolate brake boost and modulator system parts out of the equation and doing a drive first just in case...then onto the other things?.
Good luck!.
~ Pete
