potterton performa 24 problem?

i have a potterton performa 24 boiler, from cold it works great but once up to temp it switches off, pump over run on, and keeps on running and wont fire back up, left with no dhw or heating,tried turning temperature controls on boiler down etc, if i turn the room stat down to about 10-15 the heating kicks in and out ok, but cant use dhw as temp fluctuates up and down to off completely any one any ideas