Hey wthayden, welcome to the forum. Hope you get this licked. It's such a pain!
Do some searching (there's a search tab up top) and you'll find lots of discussions about exactly that. Problem is though, I don't think there's any one solution.
I still can't quite figure out how I fixed mine, even though I did it with complete success.
In mine ('71 with dual tanks) I simply replaced both of my filler hoses (large fill and smaller vent), blew out the evap canister lines with low-pressure compressed air, re-set the sending units to read correctly and replaced the "sock" filters on the end of the sending units.
In theory at least, NONE of that should have fixed my problem. But as soon as I put it all back together again, it worked and I've never had that issue crop up again.
When I installed a 23 gallon replacement tank years later, it still worked fine.
My buddy's '77 did the same thing with the rear main tank, but when I looked underneath, I could see a kinked vent line. We cut it shorter, re-attached it and away we went. Never to have that issue again either.
There was nothing visibly wrong with my hoses, either kink-wise or plugged-wise, but changing them out seemed to help.
I know that's not a for-sure answer, but it might be worth a few bucks to give it a try. But look for kinks first, like the others have said. At least that fix is free!
Good luck.
Paul