--and the intermittent nature of symptoms you've mentioned, this doesn't sound like a systemic neuropathy to me (you haven't had symptoms in other parts of the body, right?).

Others have mentioned possibilities, but have you had an MRI of the cervical spine? This sounds suspiciously like a problem there, which can absolutely produce variable symptoms in the hands. You may be a little young for arthritic bone spurs pressing on nerve roots there, but you're certainly not too young for disc bulges/herniations or other conditions that may be pressing on nerve roots or the cord itself, especially if you have a traumatic or athletic history.