Why do I lose acceleration.

1997 Dodge Grand Caravan.235,000+ miles. Starts perfectly. After being parked for a few hours, I can just tap the accelerator and it'll take off like a bullet. Then maybe after the next or second stop at an intersection, when I tap the accelerator, it has lost that acceleration. It'll move, but slow and not with the quickness.I press the accelerator more as to give it the gas to move a little faster. My concern is the lost zip. Had catalytic converter, muffler, belts, head gaskets, etc. Replaced to the tune of $2800 already. Problem still exist.I'm afraid to move out from a stop into traffic as I enter into the expressway. What could be the cause.