Weird sputtering issue

Hey all,

I have been working on a weird sputtering issue in my 71 duster for a long time.
The car will start and run fine when the engine is cold/warm.
I can stand on it when I first get it out like this and it does not sputter.
After the car warms up though, it will start to sputter and get worse the longer I drive it
and the more gas I give it. I have to roll to the side of the road and wait about 10 minutes and then it acts better but still sputters as I limp home.

I know what most of you are thinking, Vapor Lock! So here are some things
I have done to it to chase this "demon". 8)
I have raised the carburetor with some spacers, ran a return fuel line to the gas tank, got rid of the electric fuel pump and went back to a mechanical fuel pump (I hated this thing anyway - noisy), made sure that the fuel lines are not near the headers, and added a manual choke. After each of these changes, the problem still exists. Could it be something simple like the gas cap seal? I think next I will resort to rebuilding the carburetor. Maybe one of the floats has a hole in it.?? It's just weird. She will start and run fine when the engine is cold, but I don't dare drive across town unless I have time to stop along the way a few times! :pale:
Thanks!