Tri-Band vs. Dual-Band allocation?

For the RAX120, and pretty much all modern models, they have thermal sensors for all of the major components that could impact stability and can get hot. The current cooling solution seems fine.
The stability issue seems more like an SOC voltage scaling and LLC config issue if it is anything like some of the single board ARM systems. (issues like that can be fixed in firmware)
The router has multiple thermal sensors and the fan comes on long before they reach a critical temperature. In such a case, if you get improved stability from getting the temperature well below the design thermal range before the fan kicks in normally, then it often points to SOC voltage issues, e.g., if a batch of VRMs are outputting slightly lower than they should be, and need a few extra millivolts offset.

While I can't be 100% sure without actually having both a fully stable unit and one that has issues and probing the voltage rails as close to the SOC as possible using an oscilloscope, it is a potential issue, especially if a device has been undervolted, or a using a voltage curve designed to reduce thermals as much as possible, where variance in components can result in rare stability issues.

You may have even experienced issues like this on some PC motherboards, if you pushed the max stable overclock for your CPU, while trying to keep the voltage as low as possible. if the motherboard ever fails, odds are likely that applying the same overclock settings may not be fully stable on the replacement board, or it may be stable but temperatures may be 1-2C hotter due to variance in the VRM.


Heatsink for the RAX120: