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

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1990 124, 300E 2.6 (daughter)
2004 Cayenne S, Fabspeed Cat Bypass, BMC Racing Filter
2000 Lexus RX 300 (Wife)
Retired 1999 ML430