Its been a long time, but in the interests of anyone who may be looking through old posts to fix this problem, I think am now fairly sure the drama is in the 40 pin board interconnect. I recently put the CG back together and had the solenoid fuse blowing, plus some other weird and wonderful things going on pretty well as Dave described in his posts.
I did a heap of research and found out that most of the problems described are due to loss of the blanking signal.
As my boards were essentially original, I seperated the 2 boards carefully and cleaned the years of dirt off the connection area as well as inspecting for any damage. Reassembled the boards and the problem has not recurred after a fortnight of solid play.
If it happens again I will replace the pins and sockets, but as the originals had no major signs of degredation I will just play the wait and see game.
