Mesh APs Occasionally Bouncing On/Offline

[NWNSM1]

We have two MR84 APs that are meshed rather than connected to the LAN.  These are part of a K-12 outdoor sports complex.  These worked perfectly for about 18 months but last spring, they occasionally bounce connectivity.  No reason.  Sometimes there may be a few users and sometimes there are no users.  

Any thoughts on how to troubleshoot this issue?  

[PhilipDAth]

Are there any trees or plants between them that may have grown bigger over the last 18 months?

Anything new been built between them?

[NWNSM1]

Nope.  These are 10' above the roofline of a football and baseball press boxes.  The open playing fields separate them from the connected APs