Hello everyone I have a 1998 dodge ram 1500 4x4 5.9L v8 slt Laramie the truck starts up really good on the mornings but if I leave it for like 4 mins running it starts to stall ,when I am leaving my hause to go to work I shift to reverse and you can feel it jerking, I shift it to drive and it does the same thing but only when your not stepping on the gas ,and when I'm driving it hits 40MPH the rpms drop from 2100 to 1400 and it starts to jerk,and when I let go of the gas the rpm goes to 1500 and then drop right back to 1400 .I changed fuel pump ,tps,cap and rotor,spark plugs,spark plug wires any ideas of what it could be need help ASAP !!!!!!

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That's a miss. You have already done most of the work to eliminate it, next, you need to take a look at the plenum. If there is oil in there, time to fix it. Also, try disconnecting the front O2 sensor, and see if it runs any better.