Hi guys. I have an HTC One X which is still using a stock ROM and it was all good until it started going into a boot loop. It happened 2 days ago and then I managed to fix it using fastboot. Then I upgraded to the latest Android 4.2 that was recently released and it went fine. But now like an hour ago, it went into a boot loop again. I tried to do what I've done 2 days ago, but it did not solve the problem. Then I tried to use the Factory Reset option in the fastboot menu, but it failed (it showed me a progress bar for a few seconds then it shut down).

So now I do not know what to do. My knowledge about the topic is very limited to what I've googled over the past couple of days. One thing I came across while googling is that I can re-install the stock RUU which will take it back to its factory condition (which sounds like a good plan). But I'm not sure whether this is just a software issue (and what caused it in the first place) or a hardware issue (in which case what could be the problem?).

I've used the fastboot command line and got the following info about my phone:
CID: HTC__044
version-main: 4.19.707.3

Any ideas?
Have you noticed anything wrong while you were working with the phone when it booted?
Do you have root?
i think (not sure, will check) the factory reset in the bootloader actually flashes a complete ruu from boot to data, that should be saved on your phone, and it doesn't work unless you have the original recovery.
Can I have more info please…
alk wrote Have you noticed anything wrong while you were working with the phone when it booted?
Not really.
alk wrote Do you have root?
Nope. But I've flashed clockworkmod last time it went into boot loop. Basically I've done the steps described here.

Currently it's working again. Once I've recharged it for a while and turned it on again, I saw that the factory reset actually worked (although it still got the Android 4.2 rather than the version that came with it when I first bought it).

I really want to know what is wrong and how to avoid this again. Let me know if you need any other info.
Why don't you wipe the cache and delvik cache from ADB. If that doesn't work, wipe the data as well, and do a clean re-installation of your firmware.
Having flashed recovery means your phone is unlocked (hopefully htcdev token unlock), and you said you followed the post so you have flashed boot.img knowing that this is the stock rom, I would prefer to keep everything stock especially the boot partition.
this link has almost all the RUU for endeavour (HTC one X) so check which is yours and keep it in case of total failure to boot you may need to flash it later on
I couldn't find the main-version you have is it one X or one X+ or maybe it is an OTA not listed yet, anyway I trust you can get it, good luck.
Same thing happened to me just a couple of months ago. Here's how I fixed it:

Go get this rom: http://forum.xda-developers.com/showthread.php?t=1546970
Recovery: https://db.tt/Y82XOdb0
Now go into bootloader and select fastboot by pressing the power button, plug in the phone and go to next step
copy the twrprecovery file into your fastboot folder then open a cmd window from fastboot folder, "hold shift and right click then open command window here". now type fastboot flash recovery twrprecovery.img
now unplug phone and press power button once and then select second option down recovery and press power button.
when recovery loads press wipe then advanced wipe and tick ALL of the boxes and then swipe to wipe at the bottom.
plug phone back in and now go back to first screen and press "mount" then press "mount usb storage", now copy the rom, that I told you to download, onto your sdcard, when it is copied press "unmount"
now extract the boot.img from the rom zip on your pc and copy it to your fastboot folder.
in recovery press "reboot" then "bootloader.
now in command window type "fastboot flash boot boot.img" and then "fastboot erase cache". unplug phone and now press power button again and select recovery again.
press "install" and then select the rom that you copied over and swipe at the button to flash it, then reboot.
Any problems reply back.