Help!! quits when it's hot

I have a 96 cutlass ciera 3.1L v6. in july i was driving it and when i would hit the gas it would bog down like it wasn't getting enough gas then it eventually died.After that I took it to a garage and they said they don't know what wrong checked it over and said it was something electrical and have to wait until it totally dies. I took the car home and ran fine for 2 months , I changed the headlight. Now its died again. I was told its either my fuel pump or catalytic convertor.had fuel pump checked and fine, don't wanna do converter until I know for sure. Once the car cools down it usually cranks and idles for about 20 mins then once it warms up it dies again start it and quits repeatedly and leave it alone for and hr an it runs fine for 10 mins and same thing. let it sit overnight and start it and runs good for 20 to 30 mins again with no signs of problems Any ideas? thanks!.

there is a coolant temp. sensor that will cut an engine off if the temp. of the engine gets too high too quick and it wont allow it to start properly agian until it cools enough, if the sensor is faulty then it may act as an electrical problem rather then an mechanical problem. i would have that checked before trying the convertor. if not have the garage check you 'grounds' if you have a bad ground from the motor tothe battery then it would do the smae thing.
-bob