Hi - a very interesting problem indeed.
As above, the problem seems to be a nerve sliding issue - not very good advice to tell someone to give up guitar now is it?? Kinda like telling Tiger Woods he shouldn't play golf anymore!
Be careful with doing exercises without having someone supervise you. Nerves are delicate things. They don't take much abuse before they get damage then take ages to settle - as you have found out!
I would try to do the radial deviation thing but without going so far as to trigger off the tension and pain. Rather, just start doing gentle movements within a pain and tension free range.
Having the problem start at the age of 14 leads me to think that you have a growth spurt at this age (or developed gangly arms as boys of this age are wont to do!). You then probably set up a recurring irritation of the nerve that simply hasn't settled.
I would also think that your cervical stability needs work.
Find a good physio who won't stick you on machines - you need a manual therapist (manipulative therapist) - check with the Chartered society of Physiotherapists for a post-grad trained manipulative physiotherapist. This way you can be sure they might have a clue - otherwise there are plenty of good physios without this qualification!
Good luck!