The signal history shows a little more variability than I like to see. Since other wireless devices are working OK on your network the problem is not likely to be a router issue or interference that Xirrus does not detect. My suspects are the adapter and its driver, with the driver at the top of the list. Hope for a driver update. :)
Disable encryption on the router and test that way. If that works OK then try WPA-PSK encryption. (If it doesn't work OK then just resume using WPA2-PSK.)
Disable encryption on the router and test that way. If that works OK then try WPA-PSK encryption. (If it doesn't work OK then just resume using WPA2-PSK.)