Painfully slow wireless internet

My wireless internet connection gets painfully slow.  It starts off fine from a fresh boot.  But eventually slows to a crawl.

I use CNET bandwidth meter to measure the speed.  Not totally scientific, but is a good indicator.  The speed drops 50x-100x after a while

Warm reboot always fixes the problem; "Diagnose" of the wireless adapter usually does as well.

I have an XPS 8300 desktop.  DW1501 Wireless-N network card.

This machine is only a few months old; it worked fine (no connection slowdown) up until a week ago or so.

My laptop connects just fine (same wireless router, and at the same time the desktop is so slow), so I don't *think* it is the router, or my internet connection.

I checked, drivers for the wireless card are up-to-date.

I am at a loss of where to even begin to troubleshoot this.

Can anyone point me in a good direction?