Dubious: I had the same problem. At the mot the tester couldn't keep the car revs at a steady 2500 for the emission test. It would rise and drop on its own, so it failed. We even put a wooden block under the pedal to prove it wasn't a human problem. No, the car was altering the revs. It was also running at 1400rpm on the idle which was far too high.
I had already cleaned the throttle but this made no difference. I tried the basic throttle reset, but again no change. I borrowed a throttle body off my local specialists but it must have been from a different model as the throttle cable cam was a different size. As my throttle was clean, I just swapped the idle actuators over. Seemed to work for a while, but then started playing up again.
But while we were discussing the problem, one of the mechanics mentioned a valve between the intake manifold and the crankcase. There are two breather pipes: the obvious one that you see joining to the inlet just before the throttle (which is the cause of the gummed up throttles) and another which is tucked away behind the throttle body and partly hidden by the plastic engine cover. This is the "oil separator valve" and according to them has a habit of sticking. I took mine off only to find the internals were missing! Stuck a new one in and the revs dropped to 800-900. They are only £2.50 new from a dealer and worth changing (you could clean it but the cover has habit of snapping at the mounting clip and won't stay on properly again).
But the erratic idle and stalling was still there. Managed to get the car through the MOT (with one very rapid throttle reset in the middle of the emissions test!), but kept stalling in the week following. I was still running on the borrowed actuator, so put mine back and it seemed to be better. But when I put the throttle body back I connected the cable before
putting the body back on the inlet manifold. This was so I could hold the throttle closed and was sure I didn't upset the actuator. I had already taken this off and replaced it, again holding the throttle closed so that if the actuator was out, it would turn the gears back to the starting position. I had just invested in AlfaDiag and an interface, so thought I would try resetting the idle actuator as well. [My mistake was not trying the car BEFORE the diag reset, so I don't know if the manual method worked or not]. But after all this fiddling and a throttle reset, the idle was perfect!! And has been since August 2005.
I think the problem is when the throttle is opened manually with the electrics off, the actuator gets "out-of-sync". Once connected back up, the ECU thinks the throttle butterfly is open slightly and make the actuator shut it ... stalling the car. Resyncing the actuator gets everything back to normal. As I said, I don't know whether you can just take the actuator off and replace it while holding the butterfly shut or an electronic reset is needed.
At worse, the actuator gearing could be damaged as the wheel/worm gear is plastic (thanks Bosch). I once found a website where someone took one apart and cleaned all the old grease out and regreased it. But as I didn't want to damage mine (or the loan one!) I didn't try this. I can't find the website now either.
Sorry for the long winded post but I think it is solvable and without hacking wires on the back of the dials. There are some pics on my website
showing the separator valve (and most of this diatribe on page dedicated to the problem). Click on the "Idle Saga" menu link.