We have just added 6 5508 conrollers and are seeing exactly the same issue, our contollers are on 22.214.171.124 and similar to you none of the AP's are actually going down.
Have you had any luck with support as I am about to open a similar case.
We are also seeing the same thing.
Has anyone heard from support about this. We are on 10.4.2
The fix was a code upgrade on the WLC to 126.96.36.199.. I need to upgrade this again to 7.4 as we just started to see a large amount of traps for "Client with MAC address xx.xx.xx.xx.xx.xx has joined profile DDDDDD" for every device that connects. This is only happening on the WLC that we have were we enabled FlexConnect. Either way, the original WLC upgrade fixed my original post.