P300B

My 2007 Commander runs perfect when cold but after 1 1/2 hours of being on the road it begins to run horribly. In stop and go traffic the engine bucks, sometimes dies off and has a complete lack of power. The check engine light flashes, sometimes i put it in neutral and rev the engine and then it is fine for a while and the mil light goes off. Sometimes on turns the engine sputters in the intersection which has led to several close calls of being hit by the oncoming vehicle. Sometimes on the freeway, climbing a slight incline the engine sputters and i lose speed and crawl to the right lane, again sometime i put it in neutral and rev the engine, slam it in drive and sometime that works. My mechanic has had the pcm tested, changed spark plugs, coils, oxygen sensors, added extra ground wires, cleared the wiring for shorts, cleaned all engine connectors including injectors plugs and road test the vehicle with the scanner connected. Stated that other than picking up dtc P300B the data stream on the scanner showed no abnormalities. There has been no change in the performance of my Commander. I will be calling the dealer to see what can be done under my 8year/80,000 mile drive train warranty.
ENGINE,