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?...