vokera linea 28 over presure?

I have a linea 28 and when i turn the heating system on the pressure rises with the temp till it hits 70 degrees and 3.2 bar however at 3 bar the safty valve ( wich is new & I just replaced ) kicks in and it looses warter from the overflow pipe till the pressure falls below 3 bar then it stopes and is OK till it is used again and then i have to re pressurise the system from 0.5bar to 1.5each time its used.