Thanks for the quick reply.
My car has been running great until yesterday when I exited the freeway and stopped at the light. My car started to idle really rough, in fact the engine was shaking so bad that the whole car was rocking. When I went to accelerate from a stop, I had no power at all until it got up to 20mph and then everything was fine until I slowed down again. Luckily my house was just around the corner. The car in question is an 86 300e with 135,000 miles. Isn't it rare for a piston ring to just break? I would think that I would have noticed it slowly getting worse if it was just worn. Could it be anything else that I should have checked out first before I opt for a tear down or replacement?