Fixing a Saunier Duval SD 623?

I live in France but have not had any success finding an answer here.
Irregularly the boiler fails to restart when the water temperature falls. This happens more and more often. The hot water from taps works ok. Extraordinary but if one taps the middle of the lower panel the boiler starts up with a jerk. Sometimes also the start button when moved slightly has an effect. I have noticed a small spark at the bottom near the panel end??
What I have done so far: professional have been called in and have changed, the thermistors, the fan, the thermocouple solenoid -successively in different visits. I have cleaned - not very professionally - the printed circuit board. Maybe its that or the start button, but I am reluctant to go out and spend more money if that is not the reason.
If any one can help I would be most grateful