I don't know if youve already tried the simple, but my car did the same thing. It would start after sitting for a while. I would drive it a short way and it would die. It would then start later on or if I sprayed starting fluid in the carb directly.
I played with all the electrical, pulled the fuel line and turned it over spaying gas to check the pump, and finally figured out that my $2 fuel filter was mostly plugged.
It was plugged just enough to kill the motor if you drove or revved it. It let enough in to fill the bowl in the carb so it would idle fine or rev for a few minutes or so.
Like I said, you may have already checked this, but otherwise it sounds like the prob I had.