I'm inclined to check the tuning. I had a similar problem when at certain high speed, i.e. 160 km/p, it refuses to go any further despite stepping all I could on the pedal.
A tuning job, and now it won't stop until its way past 200 km/h!
I *hope* thats your problem solved.
ps. I think I'm gonna be booked in the mail for doing 150 km/h (despite braking) when Swanee and I was away celebrating her birthday weekend. The copper just looked at me and nodded his head "You're on camera ... "