when it starts it revs to 5000 then goes down to 500 to 1500 and thats all it will do

So it doesn't move? I'm surprised a 3500 would have the Hemi.

maybe somethings in the gas tank and once it starts up gets sucked around the pump. I'm assuming you're not getting any codes or theyd be listed here. I know my old oil burner would shoot up but thats cause i had to keep my foot on the pedal to keep fr stalling on a cold start. 500 rpm isn't much lower than what it should be. Does your truck still run? Is there any knocking?

Yeah I think hemis should stay in 1500s and cummins should be the only powerplant offered for 2500+

also try posting in the 3rd gen section.