I have 99 ram with a 5.2 starts up fine and after a little while the rpms start to go up and down . when I give it gas it starts popping almost like a backfire. I changed plugs that where a real pain also new wires cap rotor and checked the timing everything before it started running like crap . maybe a map sensor ? need some help with this one .

 

Unplug the front O2 sensor, see if the problem goes away.