Sorry guys, just had some minor surgery and haven't been able to continue any fault-finding. All suggestions you have made are very welcome and make practical sense.
No Cosi/Andy, the won't be a ground up resto!!
Grant, will try and give more info on when the problem occurs. Luckily it plays up when under no load as well as under load so I can work on the various suggestions you have all made in my workshop and hopefully come up with a simple solution.
Once again thanks for all your inputs.
Cheers,
GrantB