Well, after spending some time last night with a new bottle of cooling
spray, I think I can confirm that Ferdinand was on the right track
when he mentioned the capacitors C116 and C117. Applying cooling spray
to these caps made the problem disappear temporarily. As these
capacitors are right next to each other, I couldn't tell for certain
which one was at fault; or if both of them were. I'll try and source a
replacement capacitor from my old (now dead) Philips monitor -
hopefully I can find a 100uF cap of high enough voltage (160V+) to use
as a replacement. I do have a 100uF 400V capacitor, but due to its
size and the confined space around C116 and C117, I don't think it
will fit.