2 scenerios I can think of is noise, interference, and/or weaker signal cause it to drop to 11, but more probably if you are not using any security measures with your wireless network then someone else with an 802.11b wireless adapter is associating with your wireless router and it drops the speed to 11mb.  That is more probable as 54mb "g" would start throttling back to other speeds faster then 11mb before reaching 11 in the first idea I gave.  If the switch comes all of a sudden, 54 drops right to 11, the second is probably happening.  I am assuming you have a 54mb router.