1984 audi 4000 1.8 automatic

Car has had full tune up and new cat installed. Drives fine for a couple of 100 miles then with out warning the car gets sluggish, looses power and just about creaps along. Pull to the side of the road, and the car will idle fine and you can also rev the engine. Put it back in drive and its like a turtle crossing the road.

I change the plugs - which look fouled and can get another couple of hundred miles before the same thing happens.

Sounds like some sensor is going haywire and effecting the fuel air mix but I don't know for sure. Any clues??

Had a 1984 1.8 4000S with automatic myself.

The car has rather simple electronics and I don't think that it is a sensor. Fouled plugs alone would probably do it but then again it would usually affect your starting and idling as well. Is it a dry sooty fouling or is it oily? If you put in new plugs and the problem goes away for a few hundred miles, then I would say it pretty much rules out the FI system. If you were running way too rich you would see the dark smoke and you would definitely smell it. I suspect that it has something to do with the ignition system. The coil might be crapping out and giving you a weak spark under load. This could be due to a bad ignition module, defective coil or a low voltage. I had a similar problem with my Mitsubishi that turned out to be a voltage drop due to oxidation on a fusible link that provided juice to the CPU and ignition. Another source would be to check the vacuum advance, if you are not advancing you will loose lots of power. There might be other problem areas but these are the ones I can think off.