The engine has just been rebuilt and almost run-in and performs perfectly whilst on the move. The following problem has occurred twice when I'm in stuck in a traffic jam with stop/start movement and longish periods on tick-over. The car starts to develop a misfire and when revved in an attempt to clear it, it struggles to increase the revs and backfires through the exhaust very loudly and repeatedly. Eventually, the engines reduces to what sounds like 1 cylinder and dies. If then left for some 15 mins, it fires up normally and drives fine provided that a clear road is found. It sounds fuel related to me as the exhaust tailpipe tends to be overly black but I'm not sure where to check for a fault(s). It may of course be some electrical/sensor problem which only manifests itself when under-bonnet temperatures rise above those whilst on the move. I should add that the temp gauge does not display any over-heating, there is no discharge of the cooling system and the thermostatic fans switch on and off as normal.

My engine runs 803 injectors and a Bayjoo chip and prior to dismantling and re-assembly, never suffered from this fault. Obviously the extensive disturbance/disconnection of the various loom components may have caused some problem but I feel that this is an overfuelling fault causing the plugs to foul and once left to cool/dry, they return to normal. However, would this result in severe backfiring?

I should add that I changed the plugs after the first occasion to no avail.

Any suggestions/guidance would be greatly appreciated.