Main suspect is the fan if its a fan flued model closely followed by the pump. Although those are top suspects there are others so it might be time to get a registered gas installer who specialises in repairs out to it. Beware those who just shake their head at it and then try to sell you a new one.
When you say the trip switch do you mean the overheat reset button on the boiler? Or the mains circuit breaker on your consumer unit [fusebox]
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