coreboot

Please attach the .config and the coreboot logs (cbmem -1) and Linux logs (dmesg).

Thank you for providing these files. As it’s a PCIe card, please also attach the output of lspci -nn, lspci -tv and sudo lspci -vvvxx – preferably with (working) Libreboot and (problematic) coreboot.

It's entirely possible that coreboot is violating some of the power-on timings of that wifi module. This could cause it to work some of the time, but fail at other times.

To fix this, or determine if this actually is a problem, someone would have to grab the datasheet for the the chip on that module and see what the timings are supposed to be. There are probably GPIOs to control the various power and enable signals to the chip. We'd have to see when those are actually being set to tell if the spec is being violated.

This is entirely a guess on my part based on the work I've done on chromebooks getting the power-on timings for the PCIe slots to meet the specs for all of the modules we were using on a build.

It's also possible as you mentioned that there's just something wrong with your individual module that's causing it to be flaky.

I would like to add a latest update. I have been having the wifi card problem intermittently and I decided to change it.
I purchased another Atheros AR5B95 and so far the problem seems resolved (I have been testing it for a few days and the issue seems gone), supporting the idea in Martin Roth's post.