If you turn the computer on with the back removed in a darkened room, can you see a blue electrical discharge ("corona discharge") around the top of the EHT transformer?

If so, the transformer really should be replaced, but you can get special high voltage silicone sealant and fix the insulation by liberally applying silicone goop to the top of the transformer. I can't remember the exact part number for the silicone, but it's for TV repair.