My son's Asus K55N went through the same throes and in the end it was a failing HDD.

Luckily I had cloned the drive long ago so I could slip that clone drive in for a test. That test passed so a little more discovery about the ASUS BACKTRACK APP to create a true restore USB stick. Now I could slip in a new HDD (cost 55 bucks for 500GB) and restore it to factory condition.

The old failed drive was another Seagate so it went into some USB case for my son to copy their files out. (OS blown, user files were accessible.)
Note: Corrected spelling error.