Your English is very good, HolmaN, as is your description.
The fact that your audio continues without interruption makes it clear that there is nothing wrong with the power supply and grounding. The fault is with the display alone, but without removing the unit and dismantling it further diagnosis is not really possible.
The unit is quite old now, so the most common faults would be a loose ribbon cable connector to the LCD display, a cracked ribbon cable (they get brittle with age), a "dry" solder joint somewhere which has gone to high resistance (modern lead-free solders are very prone to that and were especially bad until about ten years ago) or the LCD panel itself may be failing.
Even if you diagnose the fault, and intermittent faults are always very difficult to find and fix, the unit may be uneconomical to repair.
I'd suggest that you live with the fault and when it gets much worse it will be easier to diagnose and if you're lucky it may be an easy fix. Or you may decide to buy a replacement unit.
This one is available from a trustworthy Porsche breaker in the UK but will need decoding. But they're all getting on now and can fail at any time, so you may instead decide that it makes more sense to upgrade to a modern alternative such as the Kenwood DNX4230BT and keep your current unit aside for refitting if you sell the car to someone who wishes to retain originality.
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