As a first stab here, I'd guess they're using Intel NICs or chipsets in their systems.
If that's true, have them update to teh latest drivers, available from Intel's site, or the manufacturer's site.
If it's an older Intel NIC/chipset and drivers, and you're running TKIP, you might see some MIC errors and the whole radio is shutting down for short periods.
This is normal; two MIC errors, by spec should drop the radio briefly: It thinks it's under attack .. this feature can be configured off.
IF the drivers are fairly recent, try setting the "Roaming Threshold" higher. This is also a NIC setting under the 'Advanced" tab, I think.
If that doesn't help, re-position your antennas, the humans can be getting in the way of the signal.
Also, perhaps, bring up NetStumbler and check for signal and noise levels for the channel in-use. Maybe try another channel (1, 6, and 11 only, go for the least used).
The un-moving roam is usually just old Intel drivers ... so update first, then chase signals and noise.
Good Luck
Scott
