I pulled into my driveway and shut the car off. Something just didn't feel right as the engine shut down. It had been driving just fine up to that point. When the engine was started about 30 minutes later, ran like crud. RPMs were all over the place and sounded awful. At high idle things smoothed out so I figured was not a critical problem. Engine was still hot and no MIL light yet.
Decided to drive to my buddy's to use his scan tool and try to figure what happened. The trip is about 8 miles and on a straightaway, I opened up the throttle (really stomped it). Ran at about 6000 rpm for a few seconds, remembered that there was a speed trap ahead and slowed down just in time (whew!). Came to a stop light and was expecting the engine to be bad again. Dropped down to a smooth idle at 550 rpm and has been fine ever since. Never bothered to read out any codes but I will assume that something got lodged in the EGR valve and kept it from closing properly. There were no external vacuum leaks I could find.