Glow-worm 30ci boiler water pressure problem?

i have a Glow-worm 30ci and have had constant problems with the water pressure, this often drops when the boiler is off but seems to maintain it when on. it has periods when it works fine then for no reason starts playing up. I had a plumber who replaced the pressure release valve which made no difference so replaced the pressure sensor, again no joy so left claiming he didn't now what else to do. it is now showing a fault for the pressure sensor again and i am reluctant to pay to have it replaced if it not the problem, any ideas!!!