Engine runs on when throttle is closed

I have a new issue just started, when I close the throttle the engine seems to continue to race on for a few seconds, it is like the throttle cable is sticking but when I turn the engine off the throttle seems to be snapping back fine.

Getting a bit dangerous now as I cannot stop when out riding without pulling the clutch in, not so easy when you are hanging on for dear life and trying to stop.

One thing i have noticed is that it is much better when the weather is cooler, went out last night when it was really hot and the damn thing wouldn't slow down at all when I shut off, scary.....