HouseCallAuto 11/23/2011You probably have bad fuel injectors. When you drive slow the engine runs hotter, the resistance changes in the bad injectors from the increased temperature and the car will run sluggish. So, have the mechanic who fixes your car check the resistance of the injectors first when cold and then when hot. I bet they fail the test miserably.
HouseCallAuto 11/23/2011Just to add some add'l detail. '87 through '93 GM engines are well known for running issues caused by shorted fuel injectors. Normally, this is not a first suggestion when someone has a running issue, but combining my knowledge with the fact that your vehicle fits right in there with this issue and that this injector problem is so prevalent to happen to virtually every engine during these years, the odds are slanted toward this as a cause over anything else. This issue does not exist on any other engine as a pattern failure like on the GM engines. The injector that is at fault is called a "Multec" injector and if you Google that name you will have plenty of reading about this issue. I have come upon many in my career. Identifying this as a cause before the vehicle owner has already thrown the entire auto parts store at the problem without relief is rare. Rule it out first, then continue.