We are an important organisation and have hundreds of MR46 in production. I confirm this random issue, as many others in this post. Some APs are negotiating at 100 Mbps FastEthernet, not Gigabit. After some times, they are suddenly negotiating back to Gigabit. We have Meraki switches only. When we run the cable test from the console, it always finished without any error.
We have also this issue in a new buildings, with new wiring that has been checked with a professional network tester. The UTP cable connections is not shielded, so I'm wondering if shielded S/FTP cable is adviced to help with this issue.
If it's not a wiring issue, it should be adressed and analyzed by Cisco. They are too sensitive for normal prod conditions.