Need Help 1990 90 20v Quattro
 

I have been chasing a intermittent running problem. So far I have checked the blink codes which told me that knock sensor #1 was bad.... Changed it and still have the same problem. So next I changed the ECU, then the TPS sensor and the plug wires... Same thing, while driving the car the engine check light comes on then the car slows down like the ECU goes into limp mode. Then the light will turn off and the car speeds back up with full performance. Im really confused to what would cause it to do this so any help if anyone has experianced this would be much appreciated.