pressure variations on thermaclassic f30e combi?

i put pressure to 1.5 bar , turn on the heater , the pressure rises to 2. 5 bar and the safety valve lets by , , i have replaced the safety valve, when the boiler goes off , the pressure drops down to around 0.6 -0.8 bar , where the red fault lamp comes on, an the boiler wont restart untill i increase the pressure through thefilling loop valve, i have put a jar under the discharge pipe outside an get just under half a pintof fluid. can you help. saunier duval thermaclassic f30e faned flue combinationboiler