Clocks zero on startup

My Golf 1.9 Diesel has its original battery from manufacture in December 1996 and it has never needed a charge.
About 2 years ago a strange thing happened. I started the engine from cold, heard a slight electrical arcing sound. The car started as normal but the clock had zeroed. I checked all the connections but there was apparently no problem and the starter spun healthily and apart from the slight arcing sound the car started as normal.
This has happened several times over the last 2 years and has suddenly happened 3 or 4 times over the last 10 days or so.
The only conclusion I have come to is that the battery must be coming to the end of its life but the fact that the starter is still spinning at a healthy speed makes me doubt that theory.
Has anyone got any ideas as to what may be causing this ?

I'd suspect the "ignition" switch.

I'd suspect the "ignition" switch.

Thanks boschmann - sounds very plausable

Is there anything I can do to service the ignition switch or is replacement the order of the day ?

Happy New Year

There are no adjustments or repairs, it can only be replaced.

I think I may have found a way to prevent this problem and yes the ignition seems to be the key - excuse pun.
When I turn on the ignition for a cold start the glow plug indicator light does not always show as expected. I think my clock zeroing problem occurs when I ignore the lack of a glow plug indicator light and try and start the vehicle. Nowadays if the glow plug indicator light does not come on I turn the ignition off and back on again until it does - normally just one extra off/on. The problem has not recurred..........