heating?

I have a Worcester 24i Junior combi boiler, lately when i have put the central heating on, after an hour or so it cuts out and the warning light flashes. i have to leave it for about 15 minutes before i can re start it for heating and hot water again. i have it on a low summertime setting, and reaches no more than 1 1/2 bar. if i turn it up to wintertime setting the preassure gets a lot higher and it cuts out more frequently. When the heating is not in use, the hot water runs as normal. The system was installed in April 1995. thankyou