I've had similar trouble with my pf/4 50 wm-the boiler would run fine then the main burners would extinguish (after a few minutes)while the fan etc stayed on calling for duty.Eventually the burners would fire again,for another few minutes-purely random.
The problem was solved by renewing the polymer pipes to the pressure switch-both red and black-these get hard with time and split-and the partial leaks were enough to give partial running-the boiler ends were both ruptured on the inside of the bends-impossible to see without removing the pipes.
Boiler is now as good as new-it dates to the 90's
Alec Newton
June 2007