2012 VW CC catalytic converter

My check engine came on and it was diagnosed as two sensors and the oil
pan gasket. So I had them fix the gasket and the camshaft and crankshaft
sensors. Got the car back and check engine light was still on. Said it had to do
with my catalytic converter. The car drove smoothly for about two hours and
then it started stuttering again and struggles from a stopped position. Starting
to question the diagnostics of the mechanic and dont know if the catalytic
converter being faulty and "clogged" would allow the car to drive smoothly for
two hours then go bad? Put a cleaning solution in the fuel tank for the cat and
that didn't do the trick. Dont want to be put in a domino effect with fixing part
after part.