The danger of course is that you call in a boiler engineer and he begins to replace things on spec in the hope that one of his guesses solves the problem - maybe three visits before he hits the spot at a cost of £250 a go...
Your problem symptoms strongly point to a poor electrical connection although it could possibly be mechanical I doubt that it is.
PCBs in boilers come in for a lot of criticism due to poor quality soldering and the heat cycling in the boiler case could help a poorly made joint become intermittent.
If you can take a lower cover off and gain access to the internal wiring try tapping around when the fault is apparent with a wooden spoon to see if you can find the bad place. May well be a push on connector with a bad crimp - much easier to find and fix.
If you are handy with a soldering iron consider removing the PCB to inspect it and re-do any dodgy looking joints. Take a picture with a digital camera before you disturb any wiring to give you confidence when you put it back.
I know its not your boiler but this link shows how an amateur can approach a job like this.
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
If you need help to identify a dry joint then use Google Images to search for dry joints - patronisingly (sorry) the common mistake that beginners make when soldering is to use too much solder and not enough heat.
If there are any labels that say "warranty void if seal broken" ignore them - your warranty has probably run out anyway.
Good luck...
Peccavi
February 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix plumbing.