Here's a question for you radio experts (Buck)?

I have been using AT&T cellular wireless broadband service on my laptops for
several years now.

I currently have a Sierra Wireless 881U (USB-type) wireless adapter. I
previously had a Sierra Wireless 875U (the previous-generation USB device),
both of which connected with a 10-foot USB cable so I can reposition the
device for best signal strength. Before the USB devices I had a Sierra
Wireless 860 PCMCIA adapter with a third-party external antenna on a
10-foot extension cable.

These wireless adapters will work in one of three different modes:
EDGE is the old slow network
3G and HSDPA are the fast networks

I have used all three of these devices at various times at the same site,
which is located approximately 5 miles south of downtown Fort Worth, Texas.
The building is in a low area on the campus, probably 10 feet below street
level.

Each day when I get to work, I must place my wireless device on the east
side of my cube in order to be connected to the HSDPA (Fast) network.

However, somewhere between noon and 2:00 pm every day it automatically
switches to the EDGE (slow) network. If I then reposition the wireless
device to the west side of my cube, it will then reconnect to the HSDPA
(Fast) network.

(This behavior was also true of both the previous devices, including the
Sierra 860 PCMCIA adapter - once I repositioned the device/antenna, it would
reconnect to the fast network)

Can one of you rocket scientists explain this phenomenon?