I had an issue where the engine would occasionally cut out as it warmed up, it would start again but not running on all cylinders. Give it a few minutes and all would be OK. Took it to MMC and they looked at the fuel and earthing. Now solved, with the most likely cause one of the earthing points on the basis that (a) nothing was found wrong on the fuel and, (b) earthing faults were a known issue on the Plus Six.
Why that would only be when it was warming up is a mystery to me, but it could be related to your problem I guess.
