I thought "dirty contact's" on the board or switch, leading to a dry joint(s). Dry joint's are usual suspects in intermittent faults, if suspected they should be redone as opposed to touching up.
Originally Posted by huskvarna
Then old circuit board (lead solder joints) and you applying new solder (possibly lead free). In this case, I'd suggest prostitute yourself to obtain compatible lead solder.
Then I thought this thinking is killing me and gave up on it. Have a look here might be of some use. Or look here for soldering tips.
Effin useless; I know, I'm used to it by now!