I found exactly the same thing as Dave Tosh - cracked solder joint on the relay. It solved the problem.
Probably a design weakness due to vibration.
I also noticed that the plastic holder for the PCB wasn't even screwed down - it could flap around. That (a) doesn't say much for quality control and (b) is likely to exacerbate any vibration problem.
Nick Whitehead
March 2007