If you're confident it's not fuel related, it sounds like your advance is engaging too soon, which gives you more timing advance, or too much in your case. Your vacuum canister *should* be adjustable (some OEMs aren't--but for now let's go with the assumption that yours is).
>>Quick n' easy way ---With the hose removed, stick a 3/32" (or in some cases a 3mm) hex key down in there and engage the adjustment screw. Give it two turns counter-clockwise and go for a ride. If it's better, but still not 100% cured, give it one more CCW turn at a time until it's good. If two turns cures it, go back CW a half turn and see what it does. The idea is to get as much advance as possible without getting too much. Some easy trial n' error may get you there.
>>Slower but more proper way----First, hook up a vacuum gauge with a long enough piece of hose that you can watch it while driving. Record how much vacuum the engine is actually making during off-idle acceleration, cruise, etc..especially at your problem RPMs. This is invaluable info for carb tuning as well.
Now get your vacuum tester (mity-vac or the like--if you don't have one, get one--it's a must have tool in my opinion. They usually only run around $30-$40). Hook the tester to the vacuum can and pop the distributor cap off. Start pumping the up the tester and see what the number is when the pickup module and advance plate
just starts to move. Keep pumping and see at what number the plate
stops moving. Now you know the range that your distributor is capable of adding timing. (The more vacuum that can be applied , the more timing will be added, in that range). Now compare that to the readings you got while driving, and you can now dial in your distributor's vacuum advance right there. So by delaying when the advance starts, you can limit the timing that gets added because your engine only makes so much vacuum. Clear as mud, right? In other words, if during your driving test you saw that your engine pulled, say, a max of 16 in/hg, you could adjust your can to not start engaging until 14, thus giving you very little extra advance. (It works the opposite way too, if you need more timing.)
For an example my 440 would pull about 15 in/hg at slower cruise, and increase up to 20 in/hg as cruise speed increased, and that was about as much vacuum as it could muster. So, I set my advance can to start working at about the 13" mark and this let it top out at the timing number I was after when the engine made max vacuum, which was 48 BTDC. If I adjusted the can to start pulling in any sooner, it would advance up over 50 and start to get snotty just like yours...
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