Sounds like it might be a poor soldered joint (or several perhaps). You might try to repair it before buying a new or replacement one - e-bay is a good source and there are some small commercial companies who offer re-conditioned PCBs at a fraction of the new cost on an exchange basis.
If you do decide to replace it yourself take digital camera pictures in case you get confused during the reassembly.
Search using the part number of the PCB assembly if you want to check prices.
If unable to solder find a friendly neighbour.
Although not your boiler - this string shows you what may be involved...
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
Peccavi
December 2008