Most likely caused by the crank position CONNECTOR, as mine was and two others were that I have done, very random in nature.
You could try bending the pins inside the male part of the connector to see if it grips better inside, the female part loses their sprung properties after a few years and so cases lost contact as it suffers micro arcing inside housing.
A new connector(m+f) can be bought from POLEVOLT.CO.UK
tabetha