Cisco Wireless :: 5508 Wireless APs Going Down Randomly

We have a situation where wireless access points are just going down randomly on our network. The situation is as follows:
 
We have 2 Cisco 5508 WLCs (licensed for 250 APs each, with more licenses en-route), both running software version 7.2.103.0. Then we have a large-ish number (±300) of Cisco AIR-CAP3502I-E-K9 APs connecting to these two controllers. The APs are either connected to Cisco WS-C2960S switches, or in "older" areas of the network they are connected to WS-C2950 switches. We do not have PoE switches installed yet, so all APs are powered by power injectors (AIR-PWRINJ4=).
 
Anyhaw, all the access points are in FlexConnect (previously HREAP) mode, and they all connect via trunk ports to the switches in order to get the VLAN per SSID working. Both controllers are in the same mobility group, and both are in the DNS as "cisco-capwap-controller" as they are load balancing. The APs are installed about 25m apart in most areas. There is one installation where two APs are about 10m apart, but this is due to the shape of the building in which they are installed. We have no autonomous APs deployed at all...
 
The problem is that some access points work for about a day, and then they just disassociate with whichever controller they were associated to, and then they just fall off the network completely. Both WLCs, the NCS, and the network monitoring system (WhatsUp Gold) show the affected APs as physically unreachable. Looking at the trunk port on the switch, it shows that the port is up and the line protocol is up, input rate is 0 bits/sec, and output rate is up to 70,000bits/sec. There are no input or output errors on the switch port. Here is the output from one such switch port: [code]
 
Connecting a console cable to an affected AP is fruitless, as the AP appears to be unresponsive while it is "down". Rebooting the AP, then connecting a console cable works, but then you've lost your syslogs. Looking at the Ethernet interface on the AP, it appears as if the interface is picking up a lot of errors, especially CRC errors, which would indicate a physical cable problem.
 
I got our cabling contractor out to do a cable test again, and it passes every time. We have them test both the Systimax and the Molex cable installations of affected APs, and all is OK. They use a Fluke MicroScanner² cable tester which certifies the Cat 6 cable from patch lead to flylead. No problems there, and I trust that the cable is 100% as tested.
 
So I swap the power injector of an affected AP (let's call it AP A) with an AP that is not affected by this (let's call this one AP B), and AP A goes down again after a day, or sometimes only a couple of hours. AP B remains operational as usual. If I swap AP A out with a new AP (AP C), then AP C starts showing the same symptoms after about a day, and then it also just goes down. When "suspect" AP A is installed somewhere else, then it works and stays operational for, well, 58 days now. Changing the switch/switch port also makes no difference.
 
This happens to one of the APs that are part of the "10m apart deployment", but it also happens to another AP that is completely on its own in a building far away from any other wireless source, so I cannot think that this could be the problem. I happens to an AP that is less than 100m away from its closest WLC, and it happens to an AP that is more than 350km away from its closest controller. All the other APs deployed in our remote campus (350km fibre link) are 100% OK. It happens to APs connected to the older WS-C2950 switches as well as the brand new WS-C2960S switches.
 
I am at wits end with this one. Where could the problem be? I trust the "Big Green" company that does our cable installations, and I trust the equipment they use. I also trust both Systimax and Molex to be very good products. And I do trust Cisco, as this is the only network equipment we deploy.