I have 90 vette and starts fine when the ambient temperature is around 70 degrees or cooler but when it gets around 95 and warmer the car is hard to start and then runs rough when hot.  It acts like it's vapor locked.  The fuel pump has been replaced and the fuel filter also.  The Chevy dealer had it for a day trying to find out what was wrong with it but unable.  They did check the fuel pressure to the rails and it's at 30psi.  Any ideas?