Computer is acting strange

Hey guys, 
 
I was just finishing a recording session with a client, when I bumped my computer case and it shut right off. I didn't really think anything of it at the time, but I went back down and pushed the power button -- nothing. I just built this computer a few months ago. So I took it upstairs to take a look at it, everything seemed in order, so I plugged it in on it's side and it powered up. I flipped it upright and it booted fine. So I put it back in the studio and hit the power button, the CPU fan spins once then stops, like it was doing before. I took it out of the desk put it on it's side and again it booted up. I lifted it back into place while it was running, and it's running right now. Any ideas? I have no idea what would cause this behaviour. Bizarre. 

Yes, it's been running for or a few hours now, no problem, all fans are working fine. Unless I did something wrong when I put the artic silver on, but I followed the directions closely.

I'll reseat everything later today and let you know. there is no dust in it, the case is filtered (filter is clean) and it's just been built. I really doubt it's overheating, I've never had an issue with any other computer in the same spot, so airflow shouldn't be a problem. 
 
I was almost leaning to the motherboard was shorting out. It's funny that it runs no problem on it's side, but then as soon as you stand it up that's when there's a problem. I also have an aftermarket Cooler Master heatsink on the processor, so I'm wondering if maybe I didn't seat it properly. But, it seems to be on there very well. 

I was thinking it might be the PS, I really hope it isn't -- it's brand new, and I don't have a spare to test it with :(

Loose screw shorting out the motherboard?  As much of the PIA it is, I would pull the motherboard and see if something is trapped underneath.  It would also give you a chance to make sure the MB mounts are securely installed.
 
Did you check the logs to see if there were any messages about overheating? (Control Panel, Event Logs)
 
-JJ

Loose screw and case frame short...if yer lucky.
 
If you ain't...look at where the mobo power sockets sit in relation to the case standoffs...the supports for the motherboard. Real easy to crack a mobo trace or three seating a tight power plug located in between standoffs where the mobo can flex.
 
Check your warranty if you look at this and still no joy...then open her up and see if you can duplicate the "gravity fault" by moving wire bundles, PCI cards etc. Your playing with fire here, not to mention live juice, but you know about static issues, and what's hot at killing levels and where, or you best take it to the shop and might not wanna build your own to begin with.
 
You also have some sort of protection in or at the power supply. I've seen fuses w intermittant breaks, bad voltage select switches, bad back of PS on off switches, bad front of case power switch relays, shorts where the ISA type plugs in the power bundle sit before the SATA type connectors, and then there's bad mobo power relays, not properly seated mobo power relay plugs, and of course, the notorious pinched or scraped power cable, usually associated with the drive bay.
 
Look at your expansion cards if any. I've seen the edges of PCI or ISA cards barely touching one tall component on the mobo, swapping slots was a permanent fix.
 
I'm no guru, but I've built a score or more for me, including Slack, RH, Solaris, Novell NT and DOS boxes that can be finiky with hardware and PS issues, 50 or more for an airline including HPox and Sun big iron, and tweaked maybe 3500 more on the production floor at HP.
 
You have a bad connection somewhere, and you have one clue about what makes it work or not work. Clear your head and approach at times of low stress, takes some notes as you go along, and chase it out.