Most likely a faulty clock. The clock has its own timing oscillator, so that's the place to start.
 
Hi why does my dash board time clock keep gaining time on a 2003 td4 faulty clock
Are you travelling at 88mph with1.2gigawatts of power, Marty?

As the clock is standalone I'd say best bet to replace with second hand one.
 
LOL that speed something would probelly drop of i will get another clock just about to set off down the m5 for 3 hrs but not at that speed
 

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