How can I stop my Euro Combi Styx from over pressurising?

I have a Euro Combi Styx SX20 MFFICE and it keeps over pressurising.

It starts at a normal preesure cold at just over 1 bar of pressure and then after about 30 to 45 mins of working normally all of a sudden it sounds like it boils over in the boiler itself, the pressure shoots up it makes the loudest clonking and banging and it purges itself throught PRV.

Both expansion vessels are new and pressurised and the PRV is new.

I've tried bleeding all the rads but I never seem to get any air out.

Can any one help?