Having Graphics Problems But Need Help Identifying If It Is The Graphics Card

[AcridAtrament]

Hello, I'm at my wits end with this issue and hope that some one can help me out. For the past three or four weeks this is the problem that has been occurring:
Occasionally my screen will go black, it says 'no signal' as if it were unplugged from the computer. The sound also extends on a single bit for a few seconds then cuts out. The computer remains on though and the power light does not pulse as if it were in sleep mode. I've noticed this problem occurs most often while playing a video which has lead me to believe it's the graphics card. However with the sound problem as well I'm not sure what it is.
What I used to do was just power it off manually and start it back up and it would be fine for a little while. However it's been happening so frequently I just don't watch videos on my computer anymore.

My computer is a Gateway DX4200-09, Quadcore Processor, 4GB RAM and 640GB HardDrive. The video card was bought separately, it is an NVidia GeForce 9400 GT. My Operating System is Windows 7. If I'm forgetting any specs that would be helpful please let me know.

I hope someone out there can help me figure this out. I'd rather just replace the video card if that is the problem rather than bring it in to geek squad or something to figure this out.
Thank you in advanced for any help!

EDIT: I forgot to mention, I did open up my computer and dust it out with compressed air, but that didn't seem to help.

[Transfusion]

There is a HDMI sound controller on most nVidia graphics cards. Does the onboard video work without a hitch?

[AcridAtrament]

I'm not sure, I don't know how to switch it back from the nVidia to my onboard card.

Just recently as well upon doing a hard reboot it would not start up so it pulled up the startup repair screen. I tried to repair it, it suggested a restore which I okayed but then still wouldn't restart properly. The third time I tried to restart it I skipped the repair screen and opted to startup normally, it was very slow but did start back up again.

[jason2074]

You must physically removed the graphics card to enable the onboard video. How about running in safemode?

[AcridAtrament]

It loses signal while in safe mode also. Would the best thing to do be removing the nVidia and seeing how it runs without?

[Transfusion]

Yes.