Oldsmobile/Buick Repair: 1999 Oldsmobile Silhouette, silhouette minivan, fuel pressure regulator

Question
1999 Oldsmobile Silhouette minivan 205000 miles 6 cylinder if not run for a day will start and run smoothly at idle but will  run extremely rough if gas peddle is depressed at all. Smell and backfires indicate that it is running very rich. As engine reaches operating temperature the condition smooths out. Service Engine light does not come on initially but after about an hour of driving it comes on. Had a tune-up done(new plugs and wires) light was due to bad catalytic converter (replaced) two days later identical problem occurs  
including bad catalytic converter (replaced again on warrenty) The computer flags only the Catalytic Converter which the shop tells me is being torn up by the engine running rich. Took the car to the dealer which found a loose vacuum line and air filter and reattached them but could not reproduce the problem. A few days later it happens again.
   Both shops have given up.

Answer
Hi
A rich condition should not be THAT hard to locate.
rich = too much fuel or not enough air.
I would look at the fuel pressure regulator for having failed.
I would look at the fuel pressure. I would look for a bad air filter.
A faulty air flow meter. A faulty coolant sensor telling the pcm it is -30*F will make it run rich.
There are ALOT of things, but it should not be too  hard to find.
I would locate another shop with better drivability technicians.
I hope this helps
Pawl