Sorry Guys this just dawned on me as a possible problem and it only ever happened to me after giving the bike a good vigorous wash and spraying to much water at back of motor where the CPS plug lives
I have found the CPS connector is very prone to catching water and then causing a short which makes the bike misfire and act like a rev limiter is cutting in, this can be anywhere in the Rev range. I only just thought about just then while trying to think what could have gone wrong, I hadn't thought about it before as I solved it by flooding the cup shape connecter with dielectric grease till it overflowed it only takes a couple of drops of water in this connecter to cause havoc