I have a 1984 Audi 4000 automatic that runs great when it does, but develops a severe power loss to the point that it will not move the vehicle.
Engine runs, but has now power, give it the gas and it will not take it and just sputters and farts and then stalls out.

If I go in and change the plugs it sometimes will run good for a couple of weeks and then gradually looses power until it won't go. Any suggestions?[X(]

 

Timing belt, fuel filter and pump are some obvious parts to be considered.

Does your car have an MAF? If so, get it replaced.

Have your car tested for a pressure leak - vacuum test.