88 80 Q won't run warm

[Quattro8880 at aol.com]

Well not exactly. When it gets warm enough I beleive it's vapor locking. The 
car dies and I remove the gas cap, replace it and the car starts right up. But 
now the idle bounces up and down over about a 200 rpm range.

Cool it off - restart - everything is fine until it gets hot enough again.

It's not over heating but this only happens when the car has run for 1/2 hour 
or so. In other words it's hot not warm.

Strange behavior - What could it be ?

TIA