Engine sticks at 4000 revs - why?

[bikerbob]

Hi folks
I've had ny GS500 for a year now, fitted with a Suzuki restrictor. For a few weeks now, after the bike has been going for about 1/2 hour the engine will stick at 4000 revs. Kicking it into a lower gear or turning the ignition off clears the prolem but only for a while  
Someimes it even does it when I start up (when hot). All the cables are free and I've had a look at the carbs and the throttles are not sticking.

I'm baffled :x

Any ideas??

[Kerry]

See if anything in the currently active thread idle problem...please help sounds helpful.