I have an 97 A4 quattro 1.8t with 210,000 miles on it. I have a certified audi technician who does all my maintanance. Recent services include fuel filter, air filter, timing belt and complete cooling system. I have an aftermarket blow off valve that came with the car (appears to have not been installed by a professional). At times my car "hicups" when idling or when coming to a stop. The rpms drop to about 500 and just before stalling the come back to where they should be. There has been times where it has stalled. Also, around 30-35 mph when cruising down the road the car suddenly loses power, and I feel my body moving forward, then seconds later the power comes back and I can feel the car lurch forward. This never happens on the highway and it seems to happen more frequently as time goes on. I have read that it may be bad plugs, ignition coils or vacuum lines. My mechanic said if this were the case the car would throw a code and my check engine light would come on, which has never happened (I know the light works btw). Any ideas? Thank you.