pottertopn netaheat profile 30e?

i have problems with my potterton 30e boiler wen i start it runs ok for half hour then i get static sound clicking short spell with main burner going out and on sumtimes pilot light still on but no burner i hear a drop in the fan sound sutimes no clicking ive replaced the overheat thermostat thinking this cud be the problem but no sumtimes it works fine with out trouble lik its got a mind of its own plz help me