Help me wrap my head around this!

[Prime Directive]

Ok, so my PC has stopped booting 9 out of 10 times. When it does boot though it runs rock solid every time without issue. If I power completely off, chances are I won't be able to boot the PC, but if I hibernate, I can wake the PC back up and Windows will be rock solid. In a normal boot the PC would post, I could hear some type of fan or device spinning up once and then Windows (Vista) would boot. Now the PC successfully posts, but all I get is a blank screen save an underline cursor at the most upper left and I can hear the device repeatedly spinning up and down instead of the one time like normal.
Ok, so here's what's throwing me for a loop, the fact that it won't boot most of the time but when it does it is rock solid, and also the fact that I can boot from the recovery CD and run a memory diagnostic which will always find no issues, but at the end of the diagnostic it reboots the PC and most of the time it will sucessfully boot to windows (Vista).
My gut is telling me PSU but how would that explain the almost always successful booting after the memory test? On a side note the three diagnostics I can run, Mem, CPU and HDD always come back clean.
Any thoughts or ideas greatly appreciated as it's driving me batty.

[OlyComputers]

This is usually a defective hard drive, so it's odd that it would come back clean. I would suggest:
1. open the case and re-seat all the cables.
2. reset your BIOS manually by unplugging the system and removing (then reinserting) the CMOS battery, if you're not comfortable with this the BIOS should have a "restore to factory defaults" option, it's not as good but it could do the trick.

[sgamble]

This is usually a defective hard drive, so it's odd that it would come back clean. I would suggest:
1. open the case and re-seat all the cables.
2. reset your BIOS manually by unplugging the system and removing (then reinserting) the CMOS battery, if you're not comfortable with this the BIOS should have a "restore to factory defaults" option, it's not as good but it could do the trick.

I agree with #2 here. Try that first. Then check the web to see if there is a new BIOS update for you. Then proceed to step 1 IMO.

[caperjack]

I agree with #2 here. Try that first. Then check the web to see if there is a new BIOS update for you. Then proceed to step 1 IMO.

computer boots 1 of10 tries ,what possibly could a bios flash do for it ,

[Prime Directive]

So here's the latest. The PC is a Sony that is still under warranty so I can't really get into the innards to much as that'll probably void the warranty. Had several chats/calls with their "tech support". They're thinking it's the hard drive so they're going to send out a guy with a replacement.
Here's the kicker again, even with the recovery disks it has not been able to see the hard drive lately but last night I was able with the recovery disks, see the hard drive, pull off about 200 gigs of data, do a complete wipe writing zeros to the entire drive, a complete system restore. Then I started installing apps, antivirus, office, etc. During this procedure I had to reboot the PC a bunch of times and had ZERO problems, everything rock solid. I shut the PC down for the night and of course this morning does not boot at all again, same symptoms.
I'll keep things posted, maybe something can be learned from this, got a feeling it will be ugly though.

[Prime Directive]

Wanted to say thanks to those that posted to help and also if this helps anyone with their PC issues.
So it turns out it was the hard drive, Sony tech came out, replaced it, we performed a system restore and everything seems good. I didn't think it was the hard drive because it would scan clean and when the PC did boot it was rock solid, so it this point I'm thinking that possibly the hard drive controller was going bad as opposed to the actual media. Anyway, good luck to everyone else with your issues.