jaguar combi boiler pressure?

i have a jaguar combi boiler which is giving me problems. when i have the pressure valve closed and i'm running the bath or having a shower the boiler keeps cutting out and firing up again, also with the valve closed it drops down below 0.6 which cause boiler to read the problem F0. if i open the valve up and leave it everything is ok except the reading is now at 2.9 when i have the water running and drops to 2.4 after. Can anybody please help me with this problem?