OK, problem resolved, I think I may be looking at two issues here.
After lots of messing around I connected a faulty board (it displays garbage-don't care if I destroy it) and the picture was stable !!!!!!
I tried my Beastorizer and Neo Geo, and both boards would not sync, mmmmmm.
I had originally tried a vertical game in the cab Fixeight, and it would not sync, lets try again - bingo !
There was definitely a problem as this board would not sync before, so either the replacement transistors or the HA video chip must have been duff. As a bonus, this board showed the 'curling' problem on my Hantarex, and at the operators arcade where I bought it, but on this monitor it is perfect.
Just means some boards may not work.
I may just put it back in the old centipede cab where it came from and restore it. (as long as the boad sync's up !)
Regards,
Colin.