I'm not a plumber/heating engr but i've had this fault on the same boiler, i cured it by replacing the main PCB (£95 ish).
Took about 20-30 mins with mains off.
Fault is supposedly caused by flame sensor failing, the common excuse being the spark is going down the flame sensor and damaging the components on the main board, some say bending the LHS electrode away from the other 2 may cure this. Time will tell.
The good news is replacing the PCB fixed my problem, the bad news is, i'm an electronics engr and i can't find the fix for the board on the net, but resistors and transformers don't sound like the fault to me (because the fault is intermittent and these components are either faulty, or not, but nothing in between.)
Hopefully some more knowledgeable engr will provide which components are supposedly failing and save us all a fortune.
Beaks, October 2009