Also, I would suggest using BFGMiner as a proxy instead of slush's. I've seen slush's have issues which couldn't be explained, but BFGMiner is always spot-on.
If possible, use a port other than 8332 to rule out port issues. I usually suggest folks use 8339 or something else easy to remember. Make sure the change is in blade setup, and the proxy knows to listen on the same port.
Is it true?
They have x64 builds for Windows, but not sure if they have the built-in proxy support for Windows 8x64. You could always give it a shot.
Still, I'd focus on your port and DNS settings. Make the changes and see what happens. If you have an older computer you can toss XP or Ubuntu on, even better, because you could use BFGMiner for sure.