My potterton Lynx clicking on and off every 5 secs?

Potterton Netaheat 16-22 repeatedly cuts in and out. 3-5 secs.?

I have just moved into my house and have inherited a Potterton Lynx. It completes the cycle igniting the pilot and then the main burner no problem, but then it clicks and cuts out for a split second, then reignites. It continues to click on and off every 3 to 5 seconds. It is also intermittent. I'd be grateful if anyone can tell me what could be causing this?