Pilot light goes out when i put on central heating?

I have a worcester 24i junior combi boiler, when i put the heating on it runs for an hour or so, then the pilot light cuts out. i have it on summertime setting, and reaches 1 1/2 bar, it works better on a low setting. when pilot light goes off, i cannot re-light it for another 15 minutes or so, hot water is all o.k, except for when the pilot is off. it was installed, April 1995. thankyou.