I'm new to RouterOS & when I check WebFig->Log, I only see the last handfull of entries, starting with "router was rebooted without proper shutdown." Is there an easy way to retain entries longer than a single boot?
You can either configure sending log messages to some other machine running syslog or you can configure writing logs to disk. In theory you can use the built-in storage for that but that's highly discouraged (many devices lack decently sized storage plus writing logs to NAND would considerably shorten its life time). Or you can plug some USB stick (if your RB has USB jack) and configure writing logs to that device. Those logs survive reboots. However, initial surge of logs (just after boot) will not get written to disk because of delay in disk initialization. I'm kind of disapointed that those logs are not flushed after disk for logging gets available (I might write a question to firstname.lastname@example.org
To return to your original problem: when WiFi traffic increases, also power consumed by AP device increases. cAP ac needs at least 17V power input, if voltage drops below that, it might restart. Higher power consumption means higher current and higher current means higher voltage drop on wires. While standards require quite low DC resistance of UTP cables and current of 0.5A through 30 feet of UTP cable should not cause too high voltage drop, use of sub-standard cables or non-perfect plugs and outlets or even oxidation could increase resistance of the PoE feed between ER-X and cAP ac to the point where voltage on PoEin porte of cAP ac would drop too low. I'd investigate this scenario ...