Simon:
I doubt if the high-voltage block cound cause that problem.
If the picture goes bright red with retrace lines, I would suspect
an intermittant short within the picture tube gun. My 25" TV does
that about once a week or so.
It might be a short of the red cathode with the heater. In that
case, I would try to power the heater pins with a separate supply.
A small 6.3 volt transformer might work. I have also heard of using
a wire wrapped two turns around the top core of the flyback to generate
the heater voltage.
Hope this helps...
John