Potterton performa central heating problem?

Hello, everybody.
Since I dont want to be tricked by any plumber who might visit me in the future I'm looking for help so maybe I can fix it myself.
My boiler is acting very weird. Its about the central heating. When its on the boiler keeps on initiating the flame with few seconds intervals but not always start. The water in the system is VERY hot and I just cant set the temerature. It used to heat up the water and stop for a period of time until the temp of the system drops. Now it keeps trying to start, its like constant on and off, just doesnt stop. Can it be about the temp sensor (there should be some) sending the temp to the circuit board?
Thank you for your help

andy
February 2013