Engine is difficult to start?

Having a weird problem with an '04 Chevy Impala with a 3.8 L V6 engine. Most of the time the engine starts and runs fine. The engine usually idles smooth @ ~ 750 rpms, but once in awhile the idle will pick up to ~ 2000 rpm and then drop off to almost nothing, occasionally even stall out and then the engine is difficult to start. The problem can happen when the vehicle is sitting still or when its being driven down the road. The check engine light works and it does not come on when the engine has an ******* attack, as it were. I checked for error codes and there were no pending codes either. Any idea what might cause something like this?