Lights cycling bright, dim, bright, dim .. why?

[George Harris]

'87 5000CD TQ 274,000km.

My lights; headlights, dash lights and presumable everything electrical,
have just started cycling bright to dim, and back to bright then dim,
etc. I have never noticed this before, but have had trouble starting
when the battery is out of charge. I assumed the battery was being drawn
down by the cooling fan carrying on after the motor stops.

The cycling does not appear to be dependent on motor rpm.

Any ideas?

Thanks
George