A crude way to find out wether your racquet is Head light, Even, or Head heavy is to hold your frame sideways, put your finger in the corner of the throat and let it hang off it. If it tips towards the handle its headlight. If it tips towards the head, its head heavy. If it stays fairly balance, its even balanced.
300 -320 g pretty much puts it towards a stiffer frame and midplus head size. This is probably why you're arm issues are getting flared up.
You could try a putting a leather grip on it to make it more headlight.
Otherwise check out this link, http://www.youtube.com/watch?v=OaZtxzBu0Ak
this guy is worthing listening to. He's a great coach too.