It is most likely the analog board/ flyback transformer/ some other mysterious item in the dark depths of the machine that is faulty, as fiddling with the jumpers as per the various overclocking instructions posted elsewhere has had no effect. It is now back at the original settings, as at least I know that they worked very well for a couple of years. The screen is getting progressively worse, with darkening, colour distortion and poor focus making it now completely illegible, all the time. The degeneration has been very rapid. I don't think this the result of overclocking, except perhaps that the overclocking may have stressed the components and brought them to a sticky end.

I read in various places of the need to renew solder joints as a cure for such problems, but am nervous of the electrical charges stored in the components. I guess it will have to go into the "to be fixed" pile. One of these days, perhaps, once I have learned properly how to discharge a CRT and know that I can work in there safely....