I downloaded Dawnguard and at first it played nicely, but yesterday my game froze five times in less than an hour. And today it's already froze three times within twenty minutes. I know Bethesda is glitchy so I can only assume that the Add On is to blame? The freezes only occur at loading screens so far. Any suggestions?
If you go into your system settings under memory, there should be an option for clearing your system cache. This does not affect profiles or save files, so there won't be any problems with losing things you want.
After doing that, I would recommend backing up your Skyrim save files on a USB, and then deleting all settings, patches, and DLC from the game. Then redownload the Dawnguard DLC and patches.
To dramatically improve performance, if you have the memory available, I wold also recommend installing the game on your hard drive. This would decrease loading times, as well as possibly stop the game from freezing.
Of course, there is a chance that this is an issue caused by the game or the DLC, that will need patching, but if you follow these tips, you should be able to rule out anything on your end.