Obviously a Diagnostics cartridge is the best option but in reality this is not an option.
Have you done anything lately like a modification or updrage/downgrade? (even if you think it is not related).
How easy was it to remove the Glue/MMU?.
Do you have an after market RAM upgrade fitted?
Also check for shorts to the shielding, someone before suffered with that when they has a marpet unit installed and a small peice of cardboard fixed it.
