You are half way to fixing it - if vibration can make the fault go away all you have to do is to get the lower cover off and gently tap around with something non-conductive to discover exactly which piece needs to be tapped to make it work. Here's a diagram
http://i30.photobucket.com/albums/c321/ChrisPlumber/Ideal-Classic-FF-pcb.gif
People often suspect dry joints on boiler circuit boards and if you have the confidence to take the PCB out, inspect it and resolder any dodgy joints you may save yourself some £££ but it may also be a mechanical part that needs your magic tap or any of the connectors or switches. Start with gentle tapping. Use a wooden spoon as a tapper. Take care not to electrocute yourself.
Peccavi
December 2008