It's very old. Perhaps 20 years and is single beam.

Anyway, sometimes when I turn the timebase, the trace becomes bright, thick
and fuzzy, with a width of say 4mm. When the time base is below 100ms/div,
it alternates between thickness and thiness. The input signal can still be
seen however. If I turn it on after a few weeks, the fuzziness may still be
there. But then it may suddenly revert to normal and whan I next power it
up, it's still normal.
What might be wrong and does anyone have a service manual that came with it
but I lost?

Regards,
Funky