[Down Under]

Is there anyway you can measure the high voltage? Maybe it's running too high. Worst case scenario, if your voltage is correct but it's still arcing, you can always use RTV33 high voltage silicone to cover the entire area. This stuff is also great for re coating flybacks. Not cheap though. Bob Andersen has a lot of info on this as well as rebuilding networks, using either a cluster of parts covered in epoxy, or little pieces of vero board used. Both are effective and do the job nicely.

The other consideration you may want to try is replacing the high voltage lead with a modern one. I've gone on night walks armed with a screwdriver and wirecutters many times to salvage this cable out of curbside TVs, and I use it to replace all sections of the high volt cable in my Predictas.