99 durango pcm replacement?

Truck runs like crap,idles like crap,idles somewhere between 1500rpms then it drops to like 500 rpms.When I pull up to stop light I have to keep my foot on the brake while other foot is on the gas in order to keep it from dieing.My mechanic did a major tune up,plugs,wires,dist.cap,oil and air filter.It does not act up all the time,he tested fuel pressure it tested o.k.When you hook it up to the computer all it ever reads is map sensor,tested that and it reads all good according to the manual.When you hook it up to the computer while you are driving it doesn't show any reading.Turn key method of reading codes,doesn't work.