If it’s the same as the 80 series which has 3 wires going in, it will be maintained by a permanent 12V from the battery, with the other two wires being negative and an ignition switched live. There may also be a wire from the headlamp circuit which dims the figures at night. If the permanent 12v has been lost, either externally to the clock or internally on the circuit board, you will reset whenever power is disconnected.
Have you had any water get in behind the dash from a leaky windscreen perhaps?
Best thing is to investigate and see if you have 12V positive and negative permanently connected to the clock using a multimeter set to DC volts. If so then the fault is internal, possibly as simple as a soldered joint needing resoldering.