Skyrim screen lagging, flickering

[ITstudent123]

Skyrim screen lagging and flickering after playing for a while, every time I start the game

My Platform/System:

Dual boot Win 8 in Mac oSx mcbook pro mid 2012
Nvda GT 650m Driver ver 9.18.13.4475
GeForce Game Ready Driver WHQL 344.75
Intel i7 2.3 GHZ

Skyrim tried playing on medium, high and ultra, but still same problem.
Note: Installed in USB 3.0 external HD 1 TB

 

[ITstudent123]

Note: 8 GB RAM,
tried turning of Anti Aliasing, but still not work