I narrowed it down to that unit. I may very well have buggy hardware, but it might also be related to RouterBOOT.
I replaced it with a spare 751G-2HnD, which runs 6.19, but with firmware left at 3.07. I am not taking chances for now, it's my last home unit.
Unit seems to run perfectly stable.
I cannot really do some credible debugging without some access that involves more then a few beeps and leds.
So, I used a switch in between the pc and rb951g so I could keep the links up and do an easier netinstall. Installing "fewer" packages doesn't really work. I noticed that using any other package then the upgrade npk (recommended for netinstall) makes the unit bootloop, not boot completely or behave really erratic, in cases it actually finishes boot.
Then I found random unit freezes and completely dead boot when issuing a reset-configuration. Only way out was netinstall.
Atm, I have v3.18 fw on it, but even downgrade to 3.07 does the same now.
So, is it possible to pick inside the unit via those UART pads? Unit is out of warranty, so I'm pretty much stuck with it, and I assume any responsibility of things going wrong. It is not reliable anyway until fixed, or confirmed bad, so not using it. I do am experienced in soldering and know how to use an UART. But it gives me no output whatsoever. Even when unit gets running.
[LATER EDIT] I also find it interesting that when doing netinstall on this unit and selecting Default configuration, it actually does no default config. That script is MIA. On all other RBs I worked with, when selecting Def conf in netinstall, on first boot it actually ran that default config script. Not here. Any idea why and how can this be related to my problems? Thanks in advance if anyone can drop a 50 here.