If it suddenly kicks out gear after you have shifted and fully engaged the gear, I believe that's a different problem than the syncro-gear as described. If you're having trouble getting it into the gear, then there could be a problem with the syncro-gear.
Popping out of gear is a saftey issue and should properly dealt with. As stated, become better equiped to explain what it takes to make the problem happen, so your mechanic will be more likely to witness the problem.
Make sure to document the reason for your prior visit to the dealer, in case the problem is not diagnosed until after the warrenty expires.