Gen1 losing power at top end!

[fazer390]

I went for a quick blast on Sunday. Whilst overtaking a car I hit a ton, at which point I lost power, the bike stopped pulling, the engine felt like it was choking and failed to continue accelerating! Under that speed it runs perfect.

I recently serviced the Exup and the servo still operates. I can't think what else it could be...

Any ideas???

:\