Dell Wireless 1801 - Frequent/Intermittent Speed Loss

Hi all.  First time poster here looking for some guidance on this issue.

My XPS 8900 with this network card runs extremely slow on a 802.11n 2.4GHz connection.  The signal strength is excellent (95-100%) and all other devices using the same access point are performing well. 

Another computer with 11n achieves 20Mbps Internet download (my service rating) while the XPS varies between 0.5 and 6.0 Mbps.  Occasionally, it briefly returns to full performance (20Mbps) then starts to degrade.

I tried rebooting the XPS, and the router multiple times in different sequences with no improvement. The Dell Update service is enabled and I confirmed the latest drivers are installed.

I can't correlate this behavior to any deliberate software installation or upgrade event.  It has been going on intermittently for several months.

Since the problem is intermittent, I'm guessing the hardware is OK and that some rogue service or bad setting(s) are to blame, but I don't know how to track the culprit(s).

Any ideas how to proceed?