Difficult to start, idles low?

1990 Pontiac Firebird Formula 305 mileage: 120,000. We recently acquired this car, it has high mileage but ran extremely well with a smooth idle and great pickup until last week when we had a diagnostic done on it because it failed emissions testing. The diagnostic supposedly revealed a leaking intake manifold gasket and the need for a new distributor cap, rotor, and plug wires. This car was running fine when we took it in and barely running when we picked it up. It is difficult to start, idles so low that it dies whenever you slow down or stop, and sounds like it is missing very badly when it's just sitting and idling. The place that did the diagnostic had no idea of a cause for the instant change in the way the car was running and no suggestions as to how to restore it to how it was, just that nothing they did would have caused it. We put on the new gasket (which by the way didn't look bad at all to us) as well as a couple other inexpensive parts like a thermostat and PCV valve while we had it all apart, and also put a new distributor cap, rotor, plugs, and plug wires on it as advised. Unfortunately, it is still the same as it was when we picked it up. It seems like we have checked everything but can't find anything disconnected or any cause for the low idle. We also do not understand why the computer module won't compensate for the idle because it appears to adjust other things when they are changed. Could something simple have happened when the diagnostic was done to cause this drastic change, or could it have caused a problem we can't figure out? Our inclination is to have another diagnostic done somewhere else to see what shows up but any other suggestions would be greatly appreciated.