I have seen corrosion in the crimped on cable ends do what you describe as well. You can't see it unless you pull the insulation away from the cable end and look kind of inside the connector. Just a thought on a problem I have seen that causes exactly what you are describing. Most people do what you have before finding it too.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!