Temperature variation will cause loose connections. If there are any screw connections, loosen and re-tighten them, good and tight. Inspect solder joints for cold solder or cracks. If there is a PC board, measure resistance from point to point, looking for an intermittant open in a conductor. If that does not solve the problem and there are rivited connections, drill out the rivits and re-rivit. If the equipment is old, it could be just corrosion at a rivited joint.