2000 dodge caravan sluggish and lacking power?

My 2000 dodge caravan lacks power and won't accelerate at 55 mph and under. even when stomping on the pedal. going uphills it bogs down too. Many parts have been replaced including... cat converter,o2 sensor,IAC,tps and EGR valve. still nothing. shifts into overdrive on the smallest hill. it will only accelerate if you let up on the gas. SLOOOOW on takeoff as well. fuel filter has also ben replaced. turns over 3-4 times and starts. could it be my fuel pump? or leaking injectors? I am stumped and my mechanic says nothing is wrong! beyond 60 mph it will take off but getting it there is a chore. what's next?