You freezes could be caused from an app that's behaving badly on the uSD card. Make sure you have all widget apps,apps with widgets, on the the system,not the uSD card. Once that is confirmed, make sure you put any Launchers and there associated apps you might have downloaded on to system. If still freezing, start moving any browsers back to system.
The amount of apps that you have on the uSD card will cause it to take longer for it to boot up from a power off state, but will not cause system freezes. An app that doesn't act well on the uSD card on the other hand will. An app that is corrupt or just lousy might cause the same affect.
Your 10GB of info, of which less than 1GB is probably apps(if you have less than 100 apps on the uSD card), on a 16 GB uSD card would not be considered full.
I have had some bad experiences with the Sandisk products. Has to do with the way there were designed to read info.
If after checking every app that might be suspect by deleting it then seeing if system is OK or the same,and you still are having problems. Try transferring the apps to the stock uSD card and then installing it and see if you still have the problems.
If you still have problems,then take the phone back for exchange,or call Virgin Mobile.