chaffoteaux calydra 80 problem?

hi,iv a problem with the heating,the boiler fires up to heat the rads for about 10 secs then goes off every minute or so,the fan is on constantly and the tap water is very hot,the rads slightly warm up and they sometimes reach required temp but then they cool right down then all i get is what i mentioned above(on for 10secs then off),iv changed the thermistor on the 3 way valve and still the same,..can someone please help,any suggestions much appreciated.p.s am i right in thinking maybe the tree way valve or pcb needs replacing