1997 Tahoe, automatic transmission, 5.8L V8 engine, 140K miles, engine light on. When problem occurs, will be driving at highway speeds, engine continues to run, but vehicle loses speed and must move to side of road (engine still running). The engine is turned off and re-started, chugging/clanking sound is heard/felt from rear, also lurching/surging is felt and gradually smooth/normal speed can be resumed. For background information, I have never waited with the engine running to see if normal operation is regained. I have always turned the engine off and re-started. Any suggestions what to look for?