Apple Hardware Test gives no issues, but MacBook 8,1 fails to load after login
Run Disk Utitiy from Recovery Mode. On Reboot, press Command (⌘) R. Open Disk Utility. Your partition map might be corrupt. Hit repair a couple of times.
Reset NVRAM/PRAM.  http://support.apple.com/kb/PH18761
Also run EtreCheck and post the report here. Free Download:
http://etresoft.com/etrecheck.