Computer keeps hard freezing

Lately my computer has started hard freezing. At first it was once a week, so I didn't care too much. But it has now frozen on three consecutive days. Basically I'm doing something (usually CS:GO, but also random web usage - doesn't seem to be related to high CPU/GPU load necessarily) and suddenly the image just freezes on my screen and nothing happens. No sounds, mouse doesn't work, ctrl+alt+del doesn't do anything etc.

Anyway, I believe these problems started some time after I upgraded my graphics card to a GTX 980, installed a CPU fan and overclocked the CPU. I guess the first step should be undoing the overclock, although I don't think overclock-related crashing acts like this usually.

Most of my hardware is now 5 years old. That's the PSU, mobo, CPU and RAM. The GPU is a bit under 6 months old, it's a B-stock so it should still be under warranty. What should I try first? Downclocking the CPU probably, but after that? Temperatures should not be any problem.

>i5 2500K @ 4.5Ghz.

>8GB TeamGroup Elite DDR3-1333 DIMM CL9

>550W XFX PRO550W PSU

>EVGA GeForce GTX 980 SC GAMING ACX 2.0 (B-stock)

>ASRock Z68 PRO3 mobo

I also have a second problem which is probably unrelated. I can't reboot my computer, I have to shut it down, wait for the headphones to pop (so I know it turned completely off) and then turn it on again. Otherwise I get stuck before POST at startup (I get the first 2 lines in BIOS and then nothing happens). But this has been happening for a long time, and it's not really a huge problem.

Also, my internet has started cutting off for ~15 seconds randomly. I contacted the network admins but they didn't see any problems on their end, which makes me suspect that the mobo might maybe be going bad? I'm using the integrated network card.

Edit: I have run Memtest and it didn't show any problems.