Engine running problem

We went racing this weekend and I have a fault on the machine which is proving difficult to find. The engine is pretty standard. It has a race exhaust, a 10 to 1 piston, and a big Mikuni carb fitted. She goes to 6000 and then cuts out completely, just like a rev limiter but without the buzz, firing again when she gets down to 3000. If the throttle is left open she surges up and down. We put the original carb on, no change. Changed the CDI, no change. Checked the fuel flow, enough to run a V8. Changed the coil, no change. Ran the bike with the fuel filler open, no change. Have you ever come across this problem before? Please help if you can
Later! Found the problem, had a high resistance earth leak on the ignition coil in the alternator (stator to you guys). Luckily I have an armature winder in the village who can fix it for less than an arm and a leg.