just bought 2000 durango for hunting but running rough,idling up and down,runs smooth around 1000 to 3500 rpm then backfires badly.replaced the $50. TPS yesterday but same thing.Bought wires and plugs today but didn't install them yet.my first dodge and maybe my last,too bad though,it' the magnum 5.9 fully loaded with leather and power everything.Please Help.

 

found the prob,the coil had a crack.runs like new now.