how to solve potterton flowsure+ problem?

Hi, i have a ten year old combi boiler. the central heating side of it seems to have gone a bit bonkers....its seems to come on whenever it wants too and the pressure seems to flucuate as well, one minute it'll be at 0.5 bar and the next it'll be up at 2 plus bar..I tried putting the CH switch to the off position but it still seems to come on every now and then. Any sort of help would be very much apprieciated. Many thanks