Glow-worm 30xsi boiler switch on/off?

When the boiler has reached its required temperature (as set on the Drayton RTS roomstat) it continuously switches on and off, at about a 1 second frequency. I then have to reduce the roomstat setting, and it stops (of course); after a few minutes I can increase the roomstat setting again and it works OK. Seems to be a hysteresis problem. Any tips on how to solve this problem? Thanks.