grand cherokee

I have a problem with my 97 grand cherokee. It starts fine and takes off fine but when I push on the accelerator, it loses its momentum. I almost feel it back fires. It seems to do this only when it gets hot. Sometimes it doesnt do it at all. the check engine light is on and AutoZone told me it was the sensors before and after the catalytic converter, but that was not the case. It loses alot of power going up a hill. I also just got a transmission put in.