Baxi Solo RS Overheat cut out operates - specific conditions?

I have a Baxi Solo RS boiler - about 15 years old. System starts up every morning OK about 6:30am. Heating is timed to go off at 9:00am and come on at 3:00pm. Hot Water is on all day. Frequently, in the afternoon just after the heating should come on, the overheat device in the boiler operates and shuts down system including putting out the pilot light. After manually resetting overheat and relighting, the sytem works OK. Had the boiler output temperature checked and it runs at 56C +/-4.