Baxi 80e combi loses pressure?

My baxi combi 80e loses pressure from the pressure release valve when the central heating is on. when cold the pressure is about 1 bar, rising to about 2 when heating but then after central heating switches off and cools down all the pressure goes and i notice water running out of the relase pipe. then i have to re-pressurise to about 1 otherwise the boiler makes lots of noise then goes to lock-out.