How can Iget my boiler to stay alight?

Boiler will light but, after a period of time, which, varies, the boiler will trip out.
Press the reset button and the boiler will relight but then the same thing will happen, it will trip out after a little while. However, Sometimes, it will stay alight. (Not offten though)
Haad the fan changed and the presure switch. Still no joy. Can anyone help?