It might be a component fault in the head unit itself.
If the head unit is pulled out but still connected, as far as the cables will allow, does the fault still occur? If it does, you could try locating the fault by cooling parts of the set with 'freezer spray':
https://www.amazon.co.uk/SERVISOL-20...dp/B000ZZSPRK/
You could use the freezer spray initially to determine whether or not the fault lies in the head unit. If it appears that the head unit is faulty, the simplest route might be to source a replacement head unit. Or, if you're feeling ambitious, you could continue, with the lid off the head unit, to try to locate the faulty component.
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Mark
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2002 FE S8 Ebony Black Pearl
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Cars Owned:
The Tesla Era: 2020 Model S Performance Ludicrous+ (present)
(Black, with all black premium interior and carbon fibre décor, 21" sonic carbon twin turbine wheels and FSD capability)
The Audi Era: '97 A8 4.2 (Ming Blue) --> '96 A8 4.2 QS (Dark Green) --> '02 FE S8 (present)
The Citroen Era: '84 BX 1.6 RS --> '89 BX 1.9 DTR Turbo --> '94 XM 2.0L Turbo --> '96 XM 2.0L Turbo Exclusive --> '00 Xantia Activa 2.0L Turbo
The Banger Era: '76 1.2L Lada VAZ-2101 (Ruski Fiat 124) --> '80 1.7L Morris Ital HL, finished in Ermine White and Rust