This almost always happens because your RAM isn't getting enough voltage. Did you set that manually when you were doing the overclocking, or did you just set the RAM speed and timings? I'll bet you anything those sticks of RAM take 1.6V or 1.65V and the motherboard is set to either 1.5V or "auto" by default.
This also could be happening if you have a stick of RAM that's going bad, but I'd say that's much less common, and more unlikely since the problem persists with your old RAM and with one stick at a time. If the voltage isn't what's doing it, though, I'd try running memtest86+ and see if any errors come up.
Random freezing where the power stays on and the display stays on almost always has something to do with memory, though.
1.64 is fine. ASUS has .2 increments so you can't set the exact 1.65. Run the memtest. How are your temps.? Try setting everything to default in the bios. No O.C. If it fixes then increase your O.C. slowly till the systems locks again, then back it off till stable. Have you run Prime or other tests to insure your system is stable at current settings? Components do change over time.
There are many references to random freezing on the web. The link below offers a few steps to possible solution. I would start to think the issue is software related. We always look first to memory as the source of lockups. If you rule the memory out then a software conflict is a good bet. You can remove all but one stick of memory and check if freezes happen with each single stick by swapping only one at a time in and out. So if the memory looks good, temps. are not high and everything is tight and well seated in its slots; software conflict may be the thing.
Note the second possible solution in the article. It refers to the power saving settings as a possible source. Check yours first before moving to the third issue.
Since you say the lock up has occurred at start up, I start to think it is a services conflict. The services load at start up.