Hi,
Do you mean "goes back to 12 when ignition is just switched on" or "goes back to 12 when the TD4 has been crancked over and started"?
If the latter then I would look at (clean up) the battery connections and earth connections as under starting load the battery voltage seen by the clock may drop significantly and reset the clock - sound feasable?
Could also be a battery on it's last legs giving simmilar results but you should notice a difference in the cranking speed well before the clock resets.
Hope it helps.:):)
 

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