Fps Drops

[Xazch]

Hey i recently build my new PC and have been playing Fry Cry 3 for the last 2 week when i first build the system i was getting 70-80 fps now after playing the game for a little time the fps drops to 60-50 fps i was wondering if it has anything to do with over heating at all because i run the stock Cooler on the cpu and under load am getting 77-78 Degrees Celsius

My Gpu is a R9 280x Sapphire Tri X Overclock

My Cpu is a I5 4690k

My Psu is a Coursair TxM 650

My Motherboard is a Msi Gaming 5

if you guys could give me any advice that would be helpful Edit: i also run all my games off of a 1TB WD Passport portable hard drive i have also been having issues where all my steam games on the hard drive are not being recognized but, are in the files and when i try to acess the files it says i need to use admin permissions in non order view which i then have to enable.

 

Also pic of my build in case of cable management.

http://imgur.com/sb3gl6q

Any help would be apperciatied

[Skeletonfish]

at first thought it i would thing it would be the fact that you're running games off a USB hard drive. if you feel adventurous you could pull apart the passport and put it in your build as a normal sata hard drive