Keep in mind Windows 7 is still in development.
I doubt Blizzard has any "fixes" for it.
Even your drivers may not be fully tested for Windows 7 yet.
(I see nVidia has windows 7 drivers, but with your version being in development, hard to say if they are stable.)
As it's still in test mode, It may not be fully backward compatible with WoW yet either.
You can try compatibility mode I suppose.
Most of the lockups with WoW at the loading screen are blamed on video drivers.
Also some addons can crash a log in. I was locked out of Dalaran until I disabled questhelper once. Hasn't happened since.
Could also be a background program? Something scanning or accessing the internet?
If another program "talks" while WoW is waiting for information from the server, it could get lost.
All just a few cents worth of thoughts.
Hope you find it. I know the WoW withdrawals can be murder.