My guess would be the GPS reference clock is out.

You can try by replacing the GPS receiver (it simply plugs in) - try swapping it from the good Mk3 to the new (bad) Mk4 and see if it makes a diference.

If the replacement GPS receiver doesn't fix it then the problem is on the main board - maybe there is a reference clock which is off-frequency.

You could consider sending it to VDO in Wetzler Germany for repair, or sell it for spare parts.

Regards, Jochen