Erratic Wireless Performance

I have an XPS-15 with a Killer Wireless n-1202 card running Windows 7.  I have a connection on the laptop, and initially thought it was an antenna issue.  When the computer is 20' from the router, download speeds are generally slow (around 1 Mb/s) and erratic (anywhere from 92 k/s to [rarely] 3.9 Mb/s).  For comparison, my old Inspiron in the same location gets a fairly uniform 4 Mb/s speed (generally, when I test the speed on the XPS I also do so on the Inspiron to try to control for factors like the wireless signal).  When I bring the XPS laptop next to the router, download speed is 4 Mb/s or so with little variance (generally I've been doing 10 trials in each location, using the auto download test at testmy.net since it's quick). Another thing I did initially was to disable bluetooth.  Didn't help.

Dell asked me to check safe mode, and I found that at the distant location the XPS worked fine--about 4 Mb/s with little variance.  The only things I've done to the computer are install the updated wireless driver from the Dell site and install the Citrix receiver, which I need for work.  Dell's next suggestion was uninstall Citrix, which I did; didn't help.  Next suggestion was try a clean startup:

http://www.sevenforums.com/tutorials/179159-troubleshoot-application-conflicts-performing-clean-startup.html

The clean startup didn't help--even disabling all non-Microsoft services left it with a poor < 1 Mb/s average download speed--again, with high variance.  The variance is more a problem than the relatively slow speed--when I connect to the work site, it keeps dropping.  I'm still trying to work through a solution with Dell support--that is progressing, although with time zone differences it's happening very slowly.

Does anybody have any ideas?