MMC error crashes on Banana Pro - EDIT: still unsolved

Like whaaaaaa... Okay, so long and frustrating story short, I have been having inexplicable stability issues on my Banana Pro. I have blamed the SD card, the power supply, the micro USB cable, but none of those leads seemed to actually lead anywhere.

I connected up a Fluke multimeter to the SATA power output, set to peak min mode, to see if I get any undervolting... Nope, no problem there. I ran stress tests on my sd card on my computer - it was PERFECT. I was completely stumped...

Until, after getting an Anker charging cable, and having a really good run with the new Armbian release - the system had been working for an hour without glitches, apt-get worked, I have upgraded and installed mc, ran "stress" for 20 minutes with no issues... I decided to try if Wifi will work...

(I suspected it might not, as I was (still am) getting errors like these in dmesg:

brcmf_cfg80211_reg_notifier: not a ISO3166 code (0x30 0x30)
brcmf_p2p_create_p2pdev: timeout occurred
brcmf_cfg80211_add_iface: add iface p2p-dev-wlan0 type 10 failed: err=-5

So following the "getting started" guide, I kicked off nmtui-connect, typed in my password for my 2.4 wifi ap, and then all hell broke loose. Everything fell apart right there and then, with error messages like these:
 

mmc0: card stuck in programming state!
mmc_erase: group start error -5, status 0x0
ext4fs: This should never happen
etc.

Well... that's when I looked at my board, neatly inside a Chinese enclosure, with a Chinese RP-SMA pigtail connected up to the WiFi, the cable actually touching the board... So I'm like "wait a minute right there, this can't be no coincidence"... So I removed the pigtail, re-flashed the memory card, rebooted, and for a while things were working. I upgraded the system, installed mc, tried connecting to the WiFi (didn't work without antenna), tried scanning for APs (detected the closer ones successfully)...

I was pretty happy, posted here about how WiFi interference was probably the real culprit, left the battery to charge, with the system running "stress" constantly, checking in now and then on AC voltages in /sys/power (between 4.8V and 5.1V), and CPU temp (below 61C)... and then, after two hours, I decided to install Privoxy.. and right enough, during apt-get, the MC card errors again.

Stuck in programming state, i/o error, error -5 sending status command, the whole deal. Again.

Really I don't get it.

EDIT 2: I booted up again, this time pulled off the battery to make sure that charging doesn't accidentally undervolt anything... Voltage safely above 5V according to /sys/power... Logged in again, sudo apt-get install privoxy again, mmc error again, remounted read-only, that's it.

It's an EVO32 - the SD card people keep recommending as THE card to use with SBCs... According to testing it's 100% good...