''Only you know how much time and money is cost effective to put into this project. ''Since it has booted from an external disk, sounds like there's some sort of internal disk damage (341 hrs to complete a clean install) A shade tree mchanics solution would be to''' get a replacement SSD from [http://eshop.macsales.com/Search/Search.cfm?Ntk=Primary&Ns=P_Popularity%7c1&Ne=5000&N=100007&Ntt=macbook+air+2011|OWC] and see if that solves the problem.'''
'''An alternative''' would be to ''put your orignal SSd in an external case ''and use a disk repair tool (e,g, TechTool Pro, DiskWarrior or the like) to try and repair the disk-FWIW not all disks can be repaired if there's something physically wrong with them.
Good luck,
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