Geforce GTX 660 ti performance suddenly is horrid

Hello! I have come to the forums to ask for some help that possibly somebody could help me with. I am running a 660 ti in my system for gaming, the main game that I play is Dota 2. Dota 2 isn't really a demanding game, so I expect that my GPU should be able to handle it. And for a while it has been running just fine. After a while, I decided to switch OS to Ubuntu, and began to run 3 monitors, 2 DVI and 1 HDMI from the GPU. Now I had began to experience sudden decreases of framerate within the game and the desktop while the game was runnig. I believe that it may have been because of Ubuntu, so I decided to switch back to Windows 7. Still running 3 monitors, and I have been still experiencing these issues. I have been forced to decrease my render quility and textures, which did end up helping, but is not preferable. So after a while, I decided to take out 2 of the monitors, thinking that the Gpu just couldn't handle running 3 monitors and playing Dota 2 at the same time. Yet I am still recieving problems. I have recently used a benchmarking tool, Unigine Heaven Benchmark 4.0, and was running the highest graphics. At the end, my graphics card ended up getting a score of 615 and I compared it to this review here. I have been monitor the Gpu's load with a program also while play dota, and at times it peaks at 99%.
I am running the latest Nvidia drivers, version 335.23.
My system specs are:
- CPU : Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz (3605MHz) x4
- 16 GB ram : G.SKILL Ripjaws Z Series 8GB
- PlatForm : Windows 7 (build 7601, Service Pack 1) 64bit
- NVIDIA GeForce GTX 660 Ti 9.18.13.3523 (2048MB) x1

Settings for Unigine Heaven Benchmark 4.0
- Render : Direct3D11
- Mode : 1680x1050 8xAA fullscreen
- Quality : Ultra
- Tessellation : Extreme

Any help would be nice

edit:
I did what you said Ahmadjon, and It seems to be fine right now. I am going to hook up the other 2 monitors and see if it still is fine. also, you said that the latest drivers are 337.50, but I just checked with GeForce Experience and it still says that the drivers I have now are the lastest.