Hmm... since you were able to re-install the operating system after you reformatted (a hard drive intensive activity) and it didn't lock-up I doubt it's the hard drive.
It's also probably not an overheating issue since I assume it took longer than 10-15 minutes to install the OS and it didn't freeze up then (if you wanted to double check that you could check the temps using HWMonitor
Link).
Does the hard drive activity light light up during the freeze or is it just nothing?
Also, does it still freeze up after 5-10 minutes if it's booted into safe mode?
By memory diagnoses did you mean you checked the RAM or the Hard drive?