2002 Audi A4 iM have a problem dont know if its from transm

Ok for some reason when my car is under 2,000 rpm thats when I have problems ok lets say im costing on the high way at like 45 mph the rpm's are under 2,000 the car will rev up to about 2,000 and than drop back to normal it does it about every 5 seconds. But it also does it when when I take off and the gears shif very fast I guess it does it for better gas mileage but it shifts befour 2,000 so I still have the problem and some times ill get a really hard shift caue I guess the moter does it when its about to shift and ill get a bad shake. But when its over 2,000 rpm the car runs great. Ive been trying to figure out whats wrong with it but I have no idea I dont know if its the transmission or the engine. Ive changed the sparck plugs and coils both brand new the pulgs I put in were ngk laser platinums the number for the plugs is PFR6Q. I really need help. Thanks aot