Crashes and artifacts on NVIDIA GeForce GTX 660 Ti

Hi guys! I am having serious problems with my PC and I think it has to do with my graphics card.
My PC keeps crashing, with no warning. the sound that was playing at the moment of the crash keeps repeating itself. some times I just get a black screen and then a pop-up saying that my graphics card drivers have crashed and successfully recovered. I am underclocking my graphics card with ASUS GPU tweak. These are my settings just after a blackscreen: http://tinypic.com/r/20aumaa/9 (If you can't see the picture just refresh the page and it will work).
As you can see, the GPU usage spikes after the crash of the drivers. The underclocking worked temporary, but now the crashes ar getting more frequent. I have reinstalled my drivers and my widows 3 times and the problem was just delayed. The crashes occur when I enter almost any game and sometimes I get artifacts when it crashes. If i underclock my GPU more than I am doing now, I get random flickering purple/green screens followed by the pop-up about the recovered drivers.

These are my pc specs:


Processor Intel(R) Core(TM) i5-4440 CPU @ 3.10GHz
Manufacturer Intel
Speed 3.1 GHz
Number of Cores 4
CPU ID BFEBFBFF000306C3
Family 06
Model 3C
Stepping 3
Revision 12

Video Card NVIDIA GeForce GTX 660 Ti
Manufacturer NVIDIA
Chipset GeForce GTX 660 Ti
Dedicated Memory 2048 MB
Total Memory 3.9 GB
Pixel Shader Version 5.0
Vertex Shader Version 5.0
Hardware T & L Yes
Vendor ID 10DE
Device ID 1183
Plug and Play ID VEN_10DE&DEV_1183&SUBSYS_841F1043&REV_A1
Driver Version 10.18.13.6143

Memory 8.0 GB

Operating System Microsoft Windows 7 Ultimate Edition Service Pack 1 (build 7601), 64-bit
Service Pack 1
Size 64 Bit
Edition Ultimate
Version 6.1.7601
Locale 0409

BIOS BIOS Date: 08/18/14 11:22:30 Ver: 04.06.05
Version American Megatrends Inc. F9
Manufacturer American Megatrends Inc.
Date 08/18/14

Display Maximum Resolution 1920 x 1080

Sound Device 1 Realtek High Definition Audio
Driver Version 6.0.1.7200

Sound Device 2 USB PnP Sound Device
Driver Version 6.1.7601.18208

Sound Device 3 NVIDIA Virtual Audio Device (Wave Extensible) (WDM)
Driver Version

Sound Device 4 NVIDIA High Definition Audio
Driver Version 1.3.34.4

Drive 1
Size 833.8 GB
Free 209.8 GB

Drive 2
Size 111.8 GB
Free 63.9 GB

Drive 3
Size 232.9 GB
Free 57.4 GB

Drive 4
Size 97.7 GB
Free 16.6 GB

Browsers
Internet Explorer 11.0.9600.18124
Chrome 47.0.2526.106
Firefox
Safari

Software
Java
Flash 20,0,0,0
DirectX 11.0
.NET 3.5.30729.5420
QuickTime
Adobe PDF Reader 15.9.20077.29851


I'm not that good at this, so if you could tell me what I should do, I would highly appreciate it!
Thanks in advance!