Hi I need help please.

I have a 4x4 cosworth engine.
The car runs fine all through the rev range with medium throttle.
When on full throttle and full boost, when the car reaches about 6500 revs the engine will cut out and will not respond at all - the ignition lights do not light up.
When the engine drops below appx 2500 revs the engine will then respond but will not go above 3000 revs without cutting out again.
At this point when the car is stationary when a steady half throttle is applied the engine will rev up and down from normal idle to 3000 revs. up, down, up ,down etc.
If the engine is then turned off for about 5 mins everything returns to normal until giving it full throttle again.

P.S it is not Limp Home Mode.

Any ideas will bit greatly appreciated.

Cheers Pete