random system freezes and program crashes on Lucid 64

I recently rebuilt my computer by replacing the motherboard, ram, cpu, video card, and am running Lucid 64-bit. Since the upgrade I have been plagued with system crashes every hour or so. There has been no pattern to the problem that I can see as sometimes I can go a little over a day without problems and sometimes I go 20 minutes or less. I have also not seen any pattern in my program usage that is causing the problem. The only thing I haven't tried is running my computer with nothing running as long as I can before it freezes.

I have so far encountered 4 kinds of crashes:
1) the program with focus will suddenly close
2) I will be logged off and taken back to the login screen
3) the system will freeze (when music or video is playing the sound will loop the last 2 seconds continuously) and I have to press, but not hold, the power button to shutdown my computer
4) the same as 3 except that I do have to hold the power button until my computer shuts off

I have looked around for months to find solutions and have so far tried replacing the power supply, replacing the hdd (neither solved the problem, but were necessary upgrades anyway), running memtest86+ (once with both ram cards, then once each for each of the ram cards with the other removed) which came up with nothing, disabling my screen saver, switching between Chrome and Firefox, switching between Totem and VLC, monitoring cpu and mobo temp (held constand 35C and 38C temp respectively), resinstalling X, changing from Metacity to Compiz for window management (I think that's what I did, but it was a while ago and I can't find the site that told me to do that), and "sudo add-apt-repository ppa:bryceharrington/purple" (though this came back with an error that the repository could not be found).

I am concerned that this is a a problem with my motherboard or my video card, but I don't know how to test them without replacing them and seeing what happens.

Thoughts?