1991 Audi 100 Loss of power after warm-up period

Engine Performance problem
1991 Audi 100 Front Wheel Drive Automatic 157, 00 miles

My 2.3 litre 5 cylinder Audi sat in a driveway for 2 years, starting and moving only once. It was fine until it was parked. We've addressed the fuel system issues with a new fuel pump, blown lines, new filter, etc. The car starts and runs like a sewing machine at idle for long periods of time. Under drive pressure, it's good for about 5 miles, then it starts to lose power steadily to a serious limp mode state, then dies. After cooling off completely, it will start again. Catalytic? Blocked exhaust? Injectors? Oxygen sensor? Flow sensor? I'm running out of money to throw at it and I desperately need this otherwise perfect car to run. Plenty of fuel pressure at the rail. All connections to the fuel pump have been replaced and the relays seem fine, as there's fuel pressure.