does it only happen at high rpms? if so then it is just the first phase of your synchro going bad. if you dont want to grind when you're driving "spiritedly" from second to third you should try double clutching. if you dont know what that is, it is basically clutching out of second and putting it in neutral, releasing the clutch, depressing the clutch again, up shift, and release the clutch. i hope i described it well enough for you to understand.