faulty fan relay on pcb perhaps, if fan is spinning in bursts it has sod all to do with aps and pipes etc as they are there to prove fan is working before boiler will fire up, generally pump is first in sequence so its all looking very pcb, since its been repaired twice already maybe time to bite the bullet and new one.
that would be my thoughts anyhoo, someone else may have experienced this but i aint so thats my "ahem" educated guess via t'internet
iain
August 2008