Worcester 9.24 Electronic BF boiler, pressure keeps dropping?

Hi my boiler keeps dropping the pressure, this happens straight after the central heating goes off and sometimes it just goes down after some time. this decrease in pressure causes the pilot light to go out. we normally have to put the pressure up twice. daily and put the pilot light on every 2-3 days. i think if i could sort out the pressure problem then the pilot light would stay on. after the central heating has been on, once the central heating water cools the boiler pressure drops very low-nil. also when the hot water tap is opended, the pipes vibrate and shake and the boiler flame is out. sometimes this can happen during use of warm water. please could someone let me know what the problem is and what i need to do to sort this out. this only happens during winter, during summer, the boiler is fine. also i have seen on the installation manual that the boiler has a summer/winter switch, but i dont know how this works and how to use it, maybe i could sort the problem out by adjusting this switch.
Yes before central heating is turned on, the pressure is just above 1 bar, but during central heating running, the pressure can reach 3 bars or sometimes max's out (max pressure), the water does flow out of the releife pipe. Where can i get a eplacement pressure vessel, or how can i recharge the old one?