T5400: wireless drops & cant refind network

First, this is not simply a wireless problem.

I have had 2 new T5400s with the 1505 WLAN (running XP x64, though the problem happens in Vista 64 ultimate too) and they both drop the network and stop seeing all networks after about 5 minutes.  The router is exactly 6" away, signal is fine.  The only fix is to reboot (restarting radio does not help).  After failure, the wireless fails 2 of its self tests (IRQ, DMA & Loopback).

Dell swapped out the 1505 on the first machine and the problem changed to taking 5-7 days before it occurred.

Since if I install an audio card in the same system, it too experiences failure after a period of time, I am seriously wondering if there is a fundamental flaw in the T5400 bus.  Making the problem stranger is the fact that the 1505 sits on the PCIe bus, and when the audio card is in the system it sits on the PCI-x bus.  Perhaps the problems are unrelated, but the audio card works fine in an old Dimension desktop running XP Pro 32-bit.

There must be many T5400 systems sold with the WLAN option -- do they all fail after a period of time?!  Could the problem(s) be limited to (chipset? WLAN?) drivers for 64-bit OSs?