I delivered my '92 740 non-turbo wagon to my mechanic this weekend, and it started running very poorly on the highway. I was experiencing a falling tach (1000 rpm drop or more) and severe hesitation. It's a 40 mile drive, and during the last 2/3rds of the trip, the car didn't like running at less than 60mph (I'd had the cruise set at 70mph). I parked it in his lot on Sunday afternoon, and he looked at it Monday morning. It wouldn't start, so he pushed it into the shop, and managed to get it started. Now the car runs fine (so he says) and he can't get it to act up. He says he's done absolutly nothing to the car. The reason I took it to him was the falling tach and hesitation, and my trip to his shop only confirmed my suspicions that something was drasticly wrong. He's hesitant to start replacing things since he can't find anything wrong with it, and he's ready to return it to me. I really don't want to bring it home without him doing SOMETHING to it, as I'm convinced the problem will return.

I've been reading the troubleshooting guide on the site, and I've decided to ask him to do two things: test the ignition coil, and clean the throttle body. If this does not clear things up, I'll replace the power stage/ignition amplifier myself. Am I on the right track here?