Intel i5 2500K Dying?

well 4.8 with stock voltage is imposible my friend you sure!!???i doubt that!
anyway...i see voltage flactuation..better have fixed voltage.also what about your temps man?we are talking about warer cooling arent we.?things ge tealy nasty at that voltages!also is your psu up to the task?

With no changes means you were on auto, not stock. That would have overvolted you and caused this issue. It is impossible to be at 4.8ghz at 1.21v. Not even binned cpus are that good. Don't forget your other components affect ocing as well and they all get worse with age.

LLC is causing it to go to 1.39. (VDROOP).
Degradation on sandy bridge can happen if run at over 1.4v+ over a long period I have read. What voltage was your "stock" voltage running at at 4.8 ghz?

the best possible binned 2500k can hit 4.8ghz at about 1.33-1.34v on the core under 100% load… less than 1% of them. makes me wonder what setting you were using. if one is to only change the multiplier and never touch the voltages, the cvid and motherboard automatically adjust the vcore for you.

on my gigabyte z68 the cvid will automatically adjust to 1.35 bios set voltage if i just plug in 48x, and i can boot into windows, but fail very quickly.

yes, over time after electromigration, any given chip will need more voltage to sustain clocks. this is accelerated with overclocking.

the bios has a vcore voltage like 1.35v set. what the processor is actually seeing is very different and flucuating but generally close to the bios set voltage. when the cpu goes from idle to full load, the wattage/amerage goes up causing a very quick(were talking 1/1000th of a second) drop in voltage. you notice this in your house when the lights dim when an air conditioning turns on as it takes a lot of current for a half second to start the compressor.

when you cpu goes from full load to idle quickly.. it still has incoming current but no load so the voltage will spike. vdroop is built in to protect the chip from seeing a very high voltage during this quick load change, lets say higher than 1.5-1.75v in the 2500k.

vdrop is different, it is the drop in voltage you see when there is a load compared to the voltage at idle... lets say 1.34v under full stress load vs 1.37v at idle.... compared to say a 1.35v bios setting. its hard for the chip to sustain a high clock without enough voltage to carry the current, so manually setting the voltage higher or using llc, voltage offset, etc. is required to hold the voltage to sustain the clock.

what line load calibration(llc) does is try to eleminate this vdrop so that you have a more consistent voltage, or more precisely, less of a difference between the min/max voltage between idle and load. but this is at the cost of 'vdroop' which is just a law of electronics. very good motherboards with really high quality and accurate power phases etc. are better and controlling the vdroop and also the vdrop. in most entry to mid range motherboards designed for modest overclocks, lets so no more than 5.0ghz on a 2500k, llc will reintroduce vdroop and the chip can see undetected 1/1000th second voltage spikes that only a very high quality Oscope will be able to log and see when manually tapped into the motherboard.

all in all, its not a really big deal unless your hitting your chip with high voltages... for sandy bridge, ivy bridge, and haswell, lets just say anything above 1.40v@full load, and especially if you dont have superior cooling. for me and many others... 1.35 to 1.40 is a desirable 24/7/365 setting for sandy bridge in the long term(7 years plus). me personally my 2500k is bios set to 1.38v with a certain level of llc set in and the actual voltage is 1.364 under full load and 1.406v at idle to sustain 4.8ghz, i will hit 80c with this voltage sustained in intel burn test or prime95 only though... but if i wanted to push my voltage a little higher i would need better cooling, if i want my chip to last of course. but unless i have superior cooling, i wouldn't want my idle voltage to be much higher than about 1.41v anyways so im ok with it. its been like this since april 2011.