Chrysler Repair: Unexplained misfire-engine stumble, ambient air temperature, exhaust gas recirculation valve

Question
I have a 2002 Concorde with 3.5L that has had a recurring issue with idling rough and unexplained misfire-engine stumble when accelerating from low speed. I took it to the dealer 2x to get a tune-up /check the problem. The first time (~60,000 miles) they changed the plugs,checked the engine codes (300 and I think 301 and one other) all were indeterminate for the cause. The car ran okay for about 5K miles and the problem resumed. The dealer checked the engine again, replaced the spark plugs noting two were fouled and one of them looked like it was burned, and I believe they reflashed the computer.  The car ran okay for about 6,000 miles and the same problem started to reappear. The problem is worse when the ambient air temperature is high (95+ degrees) or after the car has been running for a while with the engine at normal operating temp (slightly below midpoint on temp gauge).  The most recent time the engine only came up with a code of 300.
I would like to be able to discuss this intelligibly with the dealer's mechanic since they see the problem and treat a symptom but not the root cause.  Is it possible that the computer control system is bad?  The engine does not constantly idle rough/stumble but it seems to come and go with shorter intervals the warmer the weather is.  Note: the engine has not overtemped and I have had the regular service conducted at each recommended interval. Currently only 71,000 miles on the car.

Answer
Hi Steve,
The only part that comes to my mind, and maybe I am overly-sensitized, is the exhaust gas recirculation valve. This mechanical item can get gummed up and when it does so it can fail to close, when it must, at idle. As the result the mixture is too lean and so a miss occurs. I can imagine that it is an on-again/off-again that is random with perhaps greated likelihood as the engine warms. Checking and cleaning that valve's passageway might be the best investment of your time or money. Please let me know how this is resolved.
Roland