1998 Dodge Truck

Upon startup, after driving maybe a quarter mile the engine starts to surge & miss, sometimes backfiring. The engine will also do this if warmed up and then let sit for up to one hour and restarted. When the engine is acting up, if it is put into neutral, the rpm increased to 2000 to 3000 then put back into drive, the problem goes away. The dealer could not find any codes or anything wrong except a corroded distributer cap. New cap installed, problem still exists.