2005 sonata v6 automatic: warmed up + driving for 20-30 minutes car tachometer DROPS to 4-5 at STOPS, and either stalls or runs rough + tachometer 'fights' its way back up to correct 7-8 range.
anybody know what cause + fix are????

 

First thing I would do is check for a vacuum leak on the engine. If none exist then I would suspect the idle air controller or perhaps a balky throttle position sensor. It certainly wouldn't hurt to get the car connected to a scanner to see if any error codes reside in the ECM.