[davidimurray]

Have you tried simply leaving the car stationary and then letting it run. Get it nice and hot (wait until the fan comes on then wait 30+ minutes) and then does the fault start to occur. If so it may be related to engine bay heat. Try taking the bonnet off and runing without it - even out on the road. Where is the coil/edis/megajolt located?

When the car stops running does it still have a spark. You can get some small inline indicator lamps.Check if you have a spark on all cylinders before and during the fault condition.

If I was you I would be looking at other things before mapping. Have you got a copy of your Megajolt map ?