Ive got the same setup as you and had a very similar problem. In my case it was caused by some of the screw connectors on the back of the PID working themselves loose.
Most of the time the boiler would come up to a stable temperature, but sometimes the temperature display would drop down very low (I guess that the loose connection caused the faulty reading). Once the PID registered the low temperature it would start heating the boiler like crazy until it started reading correctly again. I didnt realise what was causing the overheating until I saw the temperature suddenly dip while I was watching it one day.
Anyway, in my case a quick tighten of the screw connectors was all it took to fix it. Hope your problem is as easily fixed!