There are some really small wires in a spring reverb which for some unknown reason sometimes break or come unsoldered. Might be the result of sudden bumps or just moving them around, who knows? Also check all the plug connections, a good cleaning of the contacts might fix the problem, it wont make it any worse. Any way it sounds like you have a weak connection or a cold solder connection someplace. Fender spring reverbs are usualy pretty reliable but like any part, they do go bad from time to time. Fortunately if they do go bad they are easy to replace and aren't too expensive, $35 to $45 US should get you a new one if all else fails, but check all your connections first, it could be something as simple as a little grease on a plug or a pin, good luck.