How can I fix my central heating system?

Th e bolier keeps tripping out!! When re-started the boiler will fire up and the 2 radiators upstairs in the main bedroom will come on and one in the bathroom. However no other radiators in the house will come on. After about 15/20 mins (times vatry) the boiler will trip out. I have to press the reset button on the boiler and keep repeating this process. Its driving me craxy and we are really desperate now!!!! I have had the pump replaced so I dont think it can be that. I thought that something must be making the boiler overheat which was triggering it to cut out but am not sure. Please, can anyone help??????