Potterton Ultra 2 BOiler problem?

My boiler and pump starts and runs for several minutes, or longer. Then it stops and goes into lockout. If I turn the thermostat on the boiler right down the pump restarts - when I turn the thermostat slowly back up the boiler starts up again, but shortly cuts out again. Sometimes the boiler/ pump go for an hour or more before cutting out - just long enough to heat the water. The same thing happens when using the central heating.

Any suggestions very wellcome.

Ian