Lion 10.7.5 constant crashing right before shut down?

I have about had it with the Mac store, so I am going to try my luck here.

I bought a Mac Pro (just the tower) about 8 months ago, and since then I have had the problem of getting the "grey" screen/crash right when it's about to shut down (when the screen goes white). I ignored it for a while, until two weeks ago it finally died and I lost everything. Sent it to the Mac store. They said they would put in a new logicboard and hard drive. Got it back. They lied to me and just reinstalled Mountain Lion (even though I had Lion before). Next day, it did it again after shutting down. Everything lost. Go back to the store. They finally do what they told me they would do in the beginning, and put in a new hard drive. Going back to work today, and as I am closing out, it crashes again! This time I did not lose my files when restarting, but I am pretty sure I will after a few reboots.

Has any one experienced the same problem, and could shed some light on how to fix this? The only extra programs I have installed are Photoshop CS6 Design Standard and Office Mac 2011.