WE HAVE FLUCTUATION ON THE PRESSURE GAUGE. HAVE REPLACED VALVE, BAXI 105E?

PLEASE COUL;D SOMEONE HELP? WE HAVE BEEN HAVING PROBLEMS WITH OUR COMBI BOILER, FIRSTLY THE PRESSURE VALVE LEAKED, AND THE CENRAL HEATING WOULD NOT WORK, UNLESS YOU TOPPED UP THE WATER. WE CHANGED THE PRESSURE VALVE, BUT NOW THE PRESSURE GAUGE IS CREEPING UP WHEN RADS ARE ON AND DROPPING RIGHT BACK WHEN THEY ARE OFF. THE PRESSURE WILL NOT STABILISE. PLEASE COULD YOU HELP, ITS COLDDDDDDD. THANK YOU, SARA. PS. ITS A BAXI COMBI 105E WITH STORAGE FOR INSTANT HOT WATER.