Why is my Mountain Lion download freezing at 53.25 MB?
There have been lots of discussion over the past few days on this, but these two threads seem to have the best details:

https://discussions.apple.com/thread/4135141?start=45&amp;tstart=0
http://forums.macrumors.com/showthread.php?t=1410290

The solution boils down to two main causes:


BT routers are known to have some kind of limit or cap on them. You'll need to find another source.
Some ISPs seem to not like the kind of traffic that is going through directly to the appstore. The solution is to pause your download, sign into a VPN or download Hotspot Shield, and start up the download process again. Once you've completed the download, you can disconnect/uninstall the VPN access.


For the record, option 2 was what I needed.