Flooding?

1986 Lincoln Towncar 200,000 miles, 302 fuel injection

When the car has been driven for awhile and I turn it off and leave it off for a short time it will not start. Starter works fine. If I leave it alone for about 30 to 40 minutes it then starts without a problem. Car runs smoothly otherwise.

I have had a complete new tune up. New fuel pump. New fuel sensor in tank. New fuel filter, air filter.

My mechanic thinks the engine for some reason is not getting fuel at these time, maybe because the fuel return line to the tank might be blocked causing a vacuum in the tank. His suggestion is to open the gas cap. Tried that, does not seem to work.

Car works fine if driven only for short trips.

Could there be a heat sensor somewhere that is malfunctioning and shutting done gas flow. I am not sure if the problem is too much gas flooding the engine or no gas getting to the engine. My mechanic thinks the engine is not getting gas.

Any one with ideas?