My 88 royale struggles in heat

Hi Guys, i am new to this so please bare with me.
I bought a 92 "88" royale in march of this year. (1st olds ever. I love the car) I am second owner of this car. I am a pizza delivery driver, and when the summer heat hits as i am driving on deliveries the car will lose accleration. I push on the gas and it bogs down, or I come to a stop and try to go again and it is slow to move no matter how far down on the gas pedal i press. Here is the kicker part, I can lift the hood for about five minutes inbetween deliveries and it runs fine again for about 20 minutes or so.
I have already replaced the thermostat last month,thinking that was the problem. it wasn't:banghead::banghead:
any ideas? Please help my boss is getting mad at me and I need to find a way to fix this. it is driving me nuts

Tough diagnosis from here, check fuel pressue first. Then suspect mass air flow sensor , but this is all guess work from here.

In addition to what Max said, could have a cat problem.....

Thank you guys for the help. Fuel pressure seems to be good. Talking with a few people in the auto motive industry they seem to have the same comments about air ratio being the cause of the problem. I tried to bounce a code and all I get the the safe start and shut off.

Gonna try replacing the airmass flow sensor as soon as I can get the $130 up to replace it. Normally i would go get a part from our local pick-n-pull but not something like this. I would rather get brand new.