Yup, its most likely your power cable and/or any any extension cord you're using. Because the case its already supposed to be grounded though the power supply (the ground cable in the 3-pin connector is mounted to the PSU case, which is touching the main casing).

But if all else fails, just attach a wire to any exposed point on your casing and shove the other end into the ground hole of the 3 pin wall socket..

i plug my cable from psu to stabilizer and stabilizer plug to the wall socket.. i think stabilizer also helps alot gua..

Ground(GND) works on steel (best use copper rod) pipe buried underground vertical.