trouble with my mk3 gti

My lovely gti has been actining out latley i just had the cat and o2 sensor replaced and it ran great for few days then started to idle very ruff!
So i bought new plug wires and a new distributer cap because it seemed to be miss firing but that solved nothing now i am at a loss. I was thinking maybe a throtle problem? or possibly the other o2 sensor/cat on the down pipe but i dont know and the mechanic is really expensive so please throw me some ideas :) thnx

oh! also it boggs very bad while on the the the throtle but if i put it to the floor it kicks back in and runs like norm