I thinks this may be it... I've been using an older WAP54G from Linksys (Access Point, not Router) and while UMA has worked for the most part it seemed to drop for no real reason. I checked the settings from OP and changed to them with the exception of the RTS Threshold. In my WAP, the value is from 256 to 2346 and the default was 2346. Changing that to one lower than recommended above, 2303 (my range is one lower so I took a stab and logic) and blam - UMA snaps right in; definitely locks in way faster. Changing that value to 2304 kills it until I change back to 2303.

Other interesting things are that the default for me for CTS protection mode is Auto and the DTIM Interval default is at 3 (range is 1-255)

This would be a cool tweak if it improves things!