Sillyspeed, I've been experiencing a similar problem on and off for a while now. Usually after about 10 mins driving and when the water temp is up to about 70deg. If I have to brake to a stop (in gear with clutch dipped)the revs will sometimes drop to zero, battery light will flash, headlights will also dip.
The car will stall unless I give the car some revs or coast a bit further. I've read all the posts about erratic idle and loss of power on acceleration etc... but non of the symptoms identify the problem that we are expeiencing.
I did have some trouble last January with my Lambda sensor at MOT, emissions failed. This was day after my 24k service at Lythgoe Fiat/Alfa of Rochdale.
I played up f*** with them as the engine diagnostic and emissions are part of the 24k service, and when they couldn't get their test equipment working on the day I returned the car, It didn't look very promising.
Eventually after I had left the car with them all day, it was returned with a new MOT issued. Their diagnosis was, that having unplugged the Lambda sensor as part of the engine diagnosis check, they must have created a fault. They cleaned up the connection (wd40 I suppose) and everything was ok.
B*ll*cks I say, MOT is due on Monday, and pound to a penny it will fail again for emissions....
If it does I'll be straight back to Lythgoe's demanding my favour off the service manager.
Alfa 156 2.5 V6 S3
1999 156 2.5 V6 Sport Pack 3 (190)
2004 GT 1.9 JTDm (150)
2007 Brera 2.4 JTDm SV (200)
2011 Guilietta 1.4 TB Multiair (170)
1970 VW T2 Early Bay (50)