1997 Plymouth Voyager Hesitation in engine

When driving my van, I have a hesitation or a feeling of backup pressure that happens once the van warms up. This problem has gone on for a month and we have changed the fuel filter, cleaned the throttle body, changed the rotor, cap, plugs, and wires. Also had the transmission fluid changed too. It still runs fine when it is cold, but after a couple of minutes it starts acting up again. I lose power that if I step on the gas, it doesn't rev up, but seems to just idle. The van has never stalled out, just power loss. Any idea's?
Thanks,
Matt