Biasi boiler high pressure problem?

I have a Biasi GARDA HE M96.28SM/B2 (combi) that keeps going to high pressure. I have replaced the expansion vessel, set correct pressure levels on the vessel and heating system and disconnected the filler loop but the pressure keeps going above 3bar and discharging out the safety valve when the boiler runs. The pressure does not reduce when the boiler cools... any idears