I'd defiantly try the tranny cocktail first but if that doesn't do anything it might be a bad syncro. I had to have mine replaced (thank God for warranty.)
You might just want to try taking it easy on the tranny though. Maybe you're cutting your 2-3 shift to short.
Probably is a bad 3rd gear syncro though. Pretty expensive to fix...tranny has to be pulled.
HTH
-neal