1997 Isuzu Trooper Check Engine Light

I have a 1997 Isuzu Tropper. I have noticed that the Check Engine Light comes on after driving for about an hours time whether it be on the highway or while I am running errands. When the light comes on it seems that there's a very slight irregularity when the car downs shifts and I also notice a thump kind of noise towards the rear of the car when I come to a stop. I have also noticed that when driving on the highway sometimes the RPM's fluxuate a little bit when the Check Light comes on. I have also noticed on two occations that when I put the car in park the RPM's go above 2000. When I park the car overnight the next morning the light is off and there are no signs of problems and it only comes on with problems stated above after driving for about an hour. I took the car into Pep Boyz for free Check Engine Light Service and the code they pulled up was P0122-TPS. During their attempt to repair, the incompitant mechanics broke of a bolt in the throttle body and said they would charge me extra to have it removed. By the way this is the second time such a problem has occured with Pep Boys. So instead of having their incompitant mechanics do any further damage to my car, I told them forget about doing the repair. The mechanic said that this was not a serious problem, but after my two previous experiences with them I just can trust a single word they say. Please HELP! Thank You.