Back again. Shop found one brush bad on the alternator, they have replaced it and are doing som extensive road tests for a couple of days to see if the problem has gone away.
So this could have been the answer Mike, thank you so much for the pointer
1990 124, 300E 2.6 (daughter)
2004 Cayenne S, Fabspeed Cat Bypass, BMC Racing Filter
2000 Lexus RX 300 (Wife)
Retired 1999 ML430