Ye sounds like the ground is poor / too far away from your location. Possibly an electrical device in the apartment "shunting" some (small) bit of current to ground as well. Since pretty much all chips nowadays are FET based; they are particularly sensitive to static. Even with just a poor ground, static alone can cause your random problems via variations in ground moisture and Relative humidity of the air. Here, too high and no static; too low and no floor conductivity. Add to this; you have created a rather nice capacitor by placing a charge carrying surface (Mobo) on a dielectric (plastic shelf), with a conductive surface on the other side (floor @ X moisture). Albeit a small value; this can still increase the likely hood of a charge accumulation without proper grounding.
If I were to make a suggestion; get a solid copper ground rod driven into earth as nearby as possible. Either through a window or a spot in the floor if possible. Either way, run lines from your earth ground directly to your GFCI. You could go a step farther and run a line from earth ground to your PSU case and mobo (screw mount hole). You could use standard solid core or stranded to GFCI and a smaller gauge insulated to PSU / MOBO; if you added those as well. Gauge isnt' as important really; as there's no real coulombs of electricity being transferred. This will guarantee real ground isolation to your critical components regardless of existing ground.
You can go even one step further, by placing a non-conductive porous material (gray foam) under the mobo; or even an anti static pad. Anti-stat pad would have a ground clip but even a porous material like the gray foam will reduce the relative surface area of the dielectric to a minimum; as compared to current plastic.
I'm pretty sure I wired the circuits correctly, but sometimes I get shocked when touch the case of the mining host computer or the ends of USB cables while standing barefoot on the cement basement floor. I also noticed that I get a slight shock if I plug my laptop in to one of the mining circuits with its 2-prong (ungrounded) power adapter.
A note, it takes about 3,000v or so; to cross an air gap and make a spark. Just to give you an idea how much difference in potential between you and the host were "floating" at that time. So say we have a power Mosfet (low RDS ON) in a closed state to ground at the moment one touches the case / board. If that person is carrying a sufficient negative charge (typical for a person), this can be discharged through the FET instead of ground; thus blowing it. Ground and poor grounds tend to represent or carry positive charges.
The ground bus bar in the circuit breaker panel for my apartment is attached to the neutral of the service entrance, and to a copper wire which is attached to the incoming copper water pipe for the house. I don't know if there's a grounding rod outside.
Older appt? At one point code would allow main grounding to water pipes. However during this time, iron pipes were used for entry and thus not the best ground long-term.
Here the rust creates a slightly semi-conductive layer that again allows a small "floating" of voltage above ground.
hope this helps
