The engine was rebuilt 2000 miles ago to standard spec apart from a fast road cam. It had new spark plugs and new magnecor leads. The car is running a bleed valve but only to reach 8psi as the amal valve was only letting the car boost to 3 psi.

The problem started occuring about 700 miles ago, after driving for a reasonable time the car would violently jerk forward when i put my foot down hard, and it reached about 5 psi of boost. Once the car did this once it will continue to till the end of the journey. Leave the car a few hours or so, even next day and the car will be fine again, may even have a journey without any problems, but once this missfiring happens once, it will do so until i stop the car.

I thought it was the MAP Sensor, i have just replaced it and on the way home it again missfired. iv have also swapped ignition packs and it has made no difference.

My question is where do i look next?

I just want to see the back of this problem now. Iv checked the sparks and all seems to be fine, apart from one which seemed to have unburnt fuel on, but cant see this has anything to do with it as it runs fine on idle before i turn it off.

Any help greatly appreciated

Dan