If it REALLY only does since you had the exhaust fitted, I would check that they made the exhaust air tight, e.g. did they renew the manifold gaskets, if the manifold tightened up etc.
Or... They might have unknowingly pulled off one of the hidden vac hoses underneath the air inlet pipes.
Also, as the guys have said above, if you change anything like that on a 16v, you technically need the CO2 screw resetting.