Hi

Have you tried any high gain antennas connected to your access point?
Or a couple of directional antennas covering your workspace.
Looking at the default antenna that comes with your access point (2db) I think at lest a 9db antenna would make more of a difference (Try before you buy tho).

What are your staff connecting with laptops, or wireless pda's?

I don’t see how the router should be going down when your downloading large files, as it should easily handle any information coming over a DSL connection. Have you tried connecting to the Ethernet ports and seeing if the router drops out when your downloading?
Try turning on the event logging on your router. Do you have all the people connecting to the router on DHCP or specified IP addresses?.

As for the use of commercial access points, I don’t know much in that area.

Joe