Can I take the internal 12V from a PC powersupply to power up a routerboard 112? Stable?
The PC is running on a UPS.
(I don’t want to use an extra 18V powersupply for the RB).
Yes, it will work 
but eventually you will ruin your PC PSU
and why is that?
However, I found the RB rebooted every few hours, perhaps the PSU isn’t stable??