99 dodge Durango 5.9 engine auto trans. I just bought this Durango and got a big surprise. It runs great but after driving it aprox. 15/20 miles it starts missing, backfiring, just running terrible and has absolutely no power. It never dies but sounds like it should. Then if you turn it off and let it set for about five minutes viola it runs great again. the only code is for a high voltage for downstream sensor 1/2. Now the guy I bought it from cut out the exaust just before the catalytic converter and removed everything to the rear of the truck. he left the O2 sensor upstream but took out every thing past that. then added a cherry bomb and nothing past that. he cut the O2 sensor off but left the wires from the connector and some of the wire from the pigtail. also he replaced the distributor cap, wires, crank position sensor, cam position sensor, engine control modual, and timing chain. nothing changed the issue.
I had to replace a idler pulley. it still runs great till you have gone 15/20 miles then it runs like garbage. Turn it off wait 5 minutes viola it runs great. 15/20 miles and crappola again. can anyone help with this nightmair I know this is fixable I just cant figure it out... Thanks in advance..

 

I'd start by putting a regular new exhaust system and all the O2 sensors (new, OEM, not cheap aftermarket).