2000 Jaguar XJR Ocasionally No Power

Electrical problem
2000 Jaguar XJR V8 Two Wheel Drive Automatic 120k k miles
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Hello,

At times, when attempting to kick alive my car... everything goes black, no electrical power at all. Doors, light...
It seems most often to happen over night, or after
a day parked.
Waiting a while (15 minutes or so, so far), opening and closing doors, etc... all come back in business.

Any clue?

mh