Help please! Frequent freezing.. / posting takes up to 5min when cold

I purchased all of my parts and assembled my computer about 4 months ago. (This isn't my first computer that I've assembled, so I have a fairly good idea of what I'm doing.) It wouldn't even post at all. Then I started swapping parts with my friend. Luckily he has the same CPU as me and after that was swapped it worked. But now, its starting to take longer to post (with his CPU still in my machine). Each time I start it up cold, it takes up to 5 minutes. But if I restart it after the delayed post it will post right away. When I reboot after posting takes too long I receive a message telling me to reset the memory and CPU frequency. When I do get into Windows, and have my computer running from anywhere between 1-6 hours, it will freeze. Any help at all would be excellent. This problem has been haunting me for a long time now.


Motherboard: Asus p5nsli
CPU: Intel Core 2 Duo E6600
Memory: 2gb Kingston 800mhz DDR2
Video Card: Sapphire Ati Radeon x1950 Pro
Power Supply: Antec Earthwatts 500w
OS: Windows XP service pack 2

I think that's everything