fan on fanfare 30/50 dosent work all the time?

I have a fanfare 30/50 boiler and the fan keeps stopping we have been told to spray the fan with wd40 and after doing so the fan then runs for a while then it stops again. The fan runs for longer if we keep the tempreture low than if it is running on high. At night we leave the boiler on at a low temp and it seems to run because the raidators are warm when we get up. Some days the fan works nearly all day then others it struggles to work at all. Were not sure if we need a new fan or if there is a way of mending the one we have.