Audi A4 B5 1996, 1.8 ADR petrol.

I've been having intermittant problems recently where the car runs absolutely fine, then runs really rough (as though it's misfiring, popping and spluttering with very little power) then it runs smoothly again, then goes through a rough patch...and so on.

I've changed the following parts already:
Spark plugs
coolant temperature sensor
air filter
throttle body (it adapted perfectly)

I took it out for a run after changing the throttle body and all appeared well (there were a couple of isolated stutters) until I sat on the drive with the car at idle...then it progressively started to run rougher and rougher, and was "missing" when I blipped the throttle, or increased engine revs slightly, then it would smooth itself out for a while before it started to run rough again.

I'm not sure where to look next.
VAG-COM shows no error codes at all.
I replaced the camshaft, and spark plug oil gasgets the other day and also checked all of the breather system...all OK.

It seems as though another sensor might be on the blink, sending wrong messages to the ECU every now and again?

Has anyone got any ideas what could be wrong?

Thanks in advance for any replies.