Ad hoc WLAN problem, adapter disconnects automatically

We set up a wireless ad hoc network with 10 machines, but some
adapter will get disconnected after some time (a few minutes to
a few hours), and there will be an event 4202 in the event viewer.
Sometimes it will show disconnected in the system tray, sometimes
it still shows connected, but the bandwidth becomes 2M and I can't
ping from or to this machine.
The configuration of this network is:
- Wireless adapter is Dell 1450 USB 2.0 54M wireless adapter.
I'm sure I'm using the latest driver.
- The adapters are operating in 802.11 G mode (ad hoc).
- The bandwidth is 54M (we also used 11M before, the same
problem happens).
- We have tried both automatically assigned IP by Windows
and static IP for each machine.
- OS is Windows XP Pro, I'm not sure about the SP version.

I have done quite a lot Googling, someone also had this problem
but no one has given a solution.

If you know the solution or any helpful information, please
tell me, thank you very much!

Xiaojun