If running 4.0 version on the controller, then some issues with 7920 handling the neighbor list when 802.11a interfaces are enabled. 4.0 implemented CCX features including neighbor list functionality.
Also some issues with MBSSID if there are many beacons present on a single channel. Will need to upgrade to 3.01 or later, but recommend 3.02 which was just released recenlty.
See the 3.01 release notes, where these issues were addressed (CSCsf06405 & CSCsf04685).
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_ipphon/english/wip7920/relnotes/rn301.htm#wp98206
Download 3.02 @ http://www.cisco.com/cgi-bin/tablebuild.pl/ip-7900ser-crypto.
Also recommend 4.0.217.0 on the controller, otherwise will need to set the Platinum qos profile's 802.1p tag to 7 vs 6 due to aging feature in the 4.0 controller release for non-WMM clients.