Ok here's the problem.

I've been having what I think is over reveing on the enginee and low performance.

The engine when changing gear's sounds as if it has a dumper valve, this is where it shounds like an over reved enginee between gears but gives you a faster aceleration between gears.
Well unfortuantly mine doesn't and doesn't have a dumper valve, there is also a hiss when changing gears or I move foot from gas pedal.

Now if it is the Intake vlave thats gone or just about gone, how easy it to replace and at what sort of price would I be looking at changing myself.

Forgot to add, that when it does this. It shounds like there is a hole in the exaust. Though there isn't, as I've checked this.