OK now I'm just confused. I went ahead and bought a 3RD wifi card (this one is a D-link DWA-556 with an Atheros chipset) to try out, and it's acting the exact same! I decided to give up for the day and boot back into Windows, and I'm having the same symptoms there!? For science, I shut down and threw my old card back in (the RNX-N300NX) to see if it was still working in Windows, but it was acting the same!

I should mention that I've also tried two different routers, so I don't think that's the issue. My laptop (Macbook Pro running OS 10.7.4) has 100% reception on my computer desk, so it's not a problem with range...wifi on my desktop was running perfectly fine in Windows before I added a new SSD and dual-booted Ubuntu.

WTF is going on?? I'm going to start a new thread since my issue seems to be bigger than the Ralink 2800 modules.