I have seen this particular fault on Cessna’s before and it has turned out to be a bad alternator on/off switch that is one half of the battery master switch.
This was the failure mode on a Mooney I used to share. The ammeter would start wobbling, and you could reach behind the panel and give the spade terminal a gentle push and the problem would go away.
Jesse, if an amateur may criticise the words of a professional:
Sure you can Jan, we all learn here don’t we? I do at least.
Maybe my description wasn’t clear. I did mean to do a runup on the ground, as it seems that the problems occurs more often than before, and if might be easy to troubleshoot at the ground. The measurement setup I suggest requires also some long wires to connect to the alternator field. Using multiple meters will prevent you to do false measurments, as you will see the problem very easy.
The alternator output should be very stable when in the regulating range. The voltage shouldn’t change with RPM when it is in the regulating range, and their is very little noise. The noise has quite a high frequency as well, so you won’t be able to measure that. The voltage changes which go as fast as the amp indicator would wobble, so that is slow enough to be visible on an analog meter.
Basically it could be anything, from poor contacts about anyware at the battery and the alternator, trough a failed regulator (common), alternator switch (common), alternator field CB (common), alternator CB (main current, also common).
I recorder wouldn’t be much of use for something like this in my opinion. Just doing a run-up with two people and some meters.
IF the problem can be reproduced on the ground, I am all with you! But reading the opening post, I see that, in spite of serious efforts, this has not been managed in four months’ time. And I am sure you know as well as myself that intermittent problems will cunningly hide as soon as serious analysis equipment comes near them, only to pop up again as soon as they see safe 
NB what’s wrong with the datalogger/datarecorder idea? Few people will have one around, but perhaps they are available for rent?
Thanks all for the input, great to have such a source of knowledge ! I’ll pass your ideas on.
@ Jan: the problem does occur on the ground (I’ve seen it myself on Saturday during the ground run as described in my post), but the mechanics are unable to trigger it. It simply comes and disappears again. I’m sure the a/c is in the shop as I write and am equally sure (not having spoken to anyone at the airport yet), that the dastardly thing behaves perfectly! As you say – the moment someone with a measuring instrument approaches, all the gremlins go into hiding…..
OK, so it looks (fingers crossed!) they found the culprit. This a/c has an overvoltage sensor. Which in turn apparently (didn’t see it myself) was
a) too sensitive
b) mounted in a position where it could be jostled by applying the hand brake and by turbulence in flight
It as now been moved and better protected. I’ll fly the a/c later in the week, so can hopefully report that all’s well and good.
Again, thanks all for your input, was much appreciated by the maintenance guys!
