C320 intermittent rough idle, help needed
 

My fathers C320 has been experiencing intermittent rough idle, almost like it's going to stall at times. But then sometimes will idle perfectly. Even when idling poorly car accelerates and runs fine. Then the check engine light came on. Finally took it to my indie mechanic, maybe not the wisest choice in this instance. He said the code was reading misfire on 2 and 4th cylinder. he has replaced the camshaft sensor and the mass airflow sensor trying to find the problem. He says the coil and wires are fine. Now he says that it may possibly be some stuck valves? I may need to bite the bullet and get the car back from him and take it the dealer. I can't find anything in searching this site that would pinpoint the problem. Any help or ideas would be greatly appreciated. This thing is costing a fortune. Thanks