I seem to be able to get a stable overclock and core unlock, with no crash or any issues.
After prolong usage under overclocking, my pc start to show a few sign of problem.

1. USB sometimes goes mad, suddenly detecting my external hdd again if I plug in another usb device (game controller)
2. Some software/games wont load, though Steam seems to be working fine. Pc will restart if software/game load.
3. CPU and GPU fan sometimes goes havoc, speeding up suddenly to max speed, usually happen when turning on the pc.
4. Final nail in the coffin, no display detected, for both onboard and pci-e. Had to hard reset mobo and remove cmos battery.

Usually it will show this issues after 1-2 weeks of overclocking. Been resetting mobo and overclock again, with many different settings, but always end up with same issues. No issues when my pc is not overclocked, but what is the point then

Could this be sign of ageing hardware? Since its nearly 3 years old, started to show issues end of last year.

Phenom II 960T (6 core unlock, 3.6ghz @ 1.4v offset, turbo core off, c1e and cool&quiet on, max temp at 52c under prime95)
Asus m4a88t-v evo
8gb ram
Sapphire 6870
Corsair HX650w

Could mobo be the main culprit?

not sure if u can adjust vcore together with cool and quiet
because it is disabled automatically

From what I've read as far as I remember last time, cool & quiet wont really disturb the vcore, and for mild overclock, c1e and cool & quiet wont really be an issues, but will help in lowering down power consumption.

Again, I could be wrong.

check your idle-load vcore using monitoring software and "cpu power management" in power option from control panel.
as long the vcore drop below stable treshold pc will crash