Wifi issues in clamshell

I realize that this has been discussed a thousand times over, but I've had my 2010 MBP for some time now, using the BookArc Stand with the MBP in clamshell mode and never noticed any degredation in Wifi speed. With that being said, it just started happening within the past few weeks...I used to get 20mbps, now I'm lucky to get 5mbps. I've tested the issue by taking it out of the stand and running it open, at which point (according to speedtest.net) I'm getting 35-40mbps.

I'm just curious if any recent Mountain Lion updates (or other) could have caused this to happen, and/or if others have only recently begun to experience the same.

Thanks for your time.