why does the pressure guage behave eratically on my baxi 105e?

when using just the hot water mode on my baxi 105e the pressure gauge remains constant but if the heating us used the pressure falls to zero after it has turned off resulting in the boiler not working when asked to start again because of low pressure if i then fill to get the pressure at say 1.5 bar it then works only for the pressure then to rise to around 3.5(but only if the heating is on) and then the process starst again