Shutter under acceleration

I have a 1997 Audi A4 1.8T Quattro. Recently I have noticed a shutter in higher gear lower to mid range RPM"s. There is no evidence in first or second throughout the band but in 3, 4 and 5 it seems to shutter from 2-5k rpm. It is bad under stress, like say accelerating up hill but not so bad otherwise. Where should I start is this transmission? Fuel supply? Valves and cam? Or something connected with the CAT and exhaust? Should I throw Turbo and intake in there as well? Thanks for any help and direction.