SMs dropping off

I have 3 SMs on an AP (5.7GHz, Advantage running 7.0.7). 2 of them seem to be dropping off. What’s strange is that the 2 that drop off appear to do so within seconds of each other.

I would think this was an interference problem, but both SMs have a signal level of -61 to -64, and linktests show that each link looks solid:

SM1:
Stats for LUID: 2 Test Duration: 2
Downlink RATE: 6543872 bps
Uplink RATE: 2230784 bps
Downlink Efficiency: 100 Percent
Max Downlink Index: 91
Actual Downlink Index: 91
Expected Frag Count: 25562
Actual Frag Count: 25562
Uplink Efficiency: 94 Percent
Expected Frag Count: 8714
Actual Frag Count: 9182

SM2:
Stats for LUID: 4 Test Duration: 2
Downlink RATE: 7150592 bps
Uplink RATE: 3513088 bps
Downlink Efficiency: 100 Percent
Max Downlink Index: 91
Actual Downlink Index: 91
Expected Frag Count: 27932
Actual Frag Count: 27932
Uplink Efficiency: 100 Percent
Expected Frag Count: 13723
Actual Frag Count: 13723

There’s nothing in the event log. After the SMs drop off, they re-register again in a minute or so. This seems to be happening every 3 or 4 hours, but only during business hours, when the SMs are actually passing traffic. Has anybody seen anything like this?