The 1.8 has a few weak spots. Cleaning the TB really is the one thing that isn't necessary on these though (get's constantly sprayed with fuel as is).
- ignition coils (as on all mk3s)
- flange between mono injector and manifold (likes to rip and let unmonitored air in)
- idle adjuster gear (plastic gears brake)
- wiring to injection (pull the rubber grommet back from the two plugs that go onto the injection unit)
- hall sender, though this is quite rare compared to other issues
I don't think this is immo related, but it would help to get fault codes anyway.