Somehow the air temperature sensor feed wire managed to get dislodged and came into contact with the edge of the blades on the second radiator fan. When the engine got really hot the fan fired up and the blades started to chaffe through the senor feed wire disrupting the signal and confusing the CPU - which affected the fueling. Once the temperature dropped the fan switched off and the exposed wire intermittently got blown/knocked against the fan housing causing ongoing false readings and bad running.
Carl at Snells figured it all out and showed me how it happened - thanks for joining in folks and 1st place to Freak who got it spot on - except Jools of course who couldn't be ar$ed
