How to fix the overheating problem on my worcester 24i rsf combi boiler?

When the boiler overheats it causes the overheating switch to pop up.

Once the overheating switch is pressed down the boiler fires up and resumes its purpose.

It gets annoying when you are having a shower and the water goes cold because of the above problem as you have to go downstairs and reset the overheat switch. This may happen a couple of times in a 10 minute period.

Sometimes the hot water can run for 20mins and it does not trip.

Could anyone advise how to sort this out.

Thanking you in advance.