Contrary to logic and what you might be told, its possible for the TDC (RPM) sensor to partially fail and give false or inconsistent readings. In this state the idiot light on the dash does not light and there is no fault in the ECU for AlfaDiag to read so all looks fine except for comedy tickover and/or very high tickover. It appears to be heat related and was fine on short trips and during the winter but got worse in the summer or on longer runs. What finally forced me to sort mine was a tickover of about 3000 while taking the kids to school, which they found amusing....
I'd already replaced the idle actuator which seemed to impove thing for a few weeks and hurt my wallet but ultimately wasn't the root cause. The good news is that the RPM sensor is a lot cheaper (perhaps £15). She's been perfect ever since.
GTV and Racing Driver