Computer help forum: Computer knocked over...

Hi,

Im new to these forums but i could really use some advice from some one savvy in this field.

About 2 weeks ago, my Gaming Desktop was knocked over. It was running at the time, as far as i know. The noticeable damage was the CPU fan had come UN-fastened form the mother board. I put it back on and it seems to be fine. its since stayed fastened and runs correctly.

When i booted it back up. there was a beeping coming from the second hard drive which i have since discovered is no longer usable. That has been removed.

Aside from that and a system repair on the first reboot, all seemed to be Ok. or so i thought.

Now no matter which game it is, i can connect to a server no problem and sit still with really no issue. As soon as i start to move i get a stuttering both on screen and in my headphones. This Lag will occur in increments. usually 5-7 second spike then 5 seconds smooth, then another 5-7 second lag spike and so on. The sound stutter matches the graphic lag i am seeing. to me is almost sounds like a disc spinning or rubbing on something.
I have removed the gfx card along with the ram and reinstalled them.
this did not help the situation. So i thought maybe the Graphics card (GTX 260 - kinda outdated) was damaged internally. I went out and purchased a GTX 550 TI.
Sadly... this did not help the problem, so i will be returning that to the store tomorrow.

So I am here seeking advice as to what you all think could be the problem. I really hope its not the mother board, and since everything runs fine until I am in game i assumed it was something easy.

what would make the most sense to test next?

HardDrive - main harddrive with OS on it and all essential files.

PSU - seems fine, although i read it could possible be the problem if damaged some how. (corsair 850watt)

Mother board. - FXF mobo, no obvious damage spotted. i guess it possible that there is a crack maybe where the gfx card connect to the pci slot. i looked at it pretty thorough.

any advice or opinions would be greatly appreciated.
like i said the computer runs fine until i get into a game.
if its too serious i would almost rather order and build a whole new rig. as of now this is a great e machine...