1990 Toyota Pickup 2WD shop told me my ignition timing was way advanced

1990 Toyota Pickup, 2WD mileage: 165,000. I live in CA and this truck has always passed smog with flying colors. It still does as far as emissions are concerned, and runs well too. However, the smog test shop told me my ignition timing was way advanced, and I confirmed it (about 25 degrees). When I adjust it to spec. (5 degrees with the check pins shorted), it barely runs. The timing has never been touched before. How could it run so lousy with the correct timing? Is there something else that has stretched (timing chain?) or is bad, like the throttle position sensor? No engine warning lights have ever come on.