I had exactly the same conditions.
The plumbers were useless. Most wanted to fit a new boiler!
My information was that a dry joint on the pcb was the problem. Dry joints are produced by incorrect soldering techniques. They may be visible but often are not. The joints near the plug conectors were badly fractured.
The time honoured procedure in the electronics field is to heat up each joint. I avoided those joints connecting chips, which can be damaged by heat. It takes about an hour. Make sure the iron is hot. Hold the iron on the joint. The joint will melt and change shape. Take away the tip of the iron immediately. Don't heat solder on the tip of the iron and let it run onto the joint. Only hold the edges of the board. Static can damage the chip.
I refitted ithe board with very little faith that it would work.
It did work. No flashing red light of death. Wonderful!
Give it a try. You have nothing to lose. Good luck.
Spike Norman
November 2009