Ferroli F30 water pressure problems?

I have a Ferroli F30 Cobi Boiler that has been working fine throughout the summer providing hot water. I have just started the heating up and have a water pressure problem. When running heating the water pressure gauge increases 4+bar. I have tried taking water from the system and have bled all of the system but problem persists. Also when system is off the water pressure gauge drops slowly until it hits zero. then the boiler will not work until pressure topped up. Any suggestions please