Unknown reboot

[templis]

Hi, I found good specs here, so decided to ask about my girlfiends PC.
It is quite old about 4 years. It was complected with:
256 MB RAM Kingstone DDR333
80GB HDD
AMD Athlon XP about 1,7 GHz
CD-ROM
Win XP Pro
I added extra 250GB HDD, DVD-ROM and after a month a 1GB RAM DDR400 A-data
When added extra memory, PC started to reboot without bluescreen. No matter, what am I doing - playing online games or writing Word document. No special things doing on PC. I thought at first it's the memory, who makes it reboot, so I changed it with a 1GB Kingstone DDR400, no changes. I removed the 256MB RAM, the same.. Reboot after a while.. I replaced it with a 512 RAM of my PC, a bit rare rebooting, but still not good. I changed in BIOS from DDR400 to DDR333, a bit more rare reboot, but still it reboots.. Sometimes 4 times in 1 hour, sometimes few times a week. I changed motherboard with bether one.. the reboot now is more frequently. So, what could be the broblem? HDD and RAM errors should be with bluescreen, maybe it's the Power suply? Maybe due to PC upgrade (RAM 2x more) the system needs more power to suply new resources of it?
Thanks!
Sorry for my english!

[so0ky]

It might be because your RAM sticks are at different frequencies.

Try to use RAM that is the same number. So DDR333 RAM sticks with DDR333 RAM. Make sense?

[bucksfanjimj]

What is the make and model # of the board that you currently have in the computer? Sounds like you have compatability issues.

[templis]

Changed the Power Supply with "FORTRON CASE PSU ATX 350W 12CM/ATX-350PNF RO" ( http://m79.lv/product/single/group/7..._ATX_350PNF_RO ) for about 28.33 USD, everything works just perfect. No reboot now for 3 days. Running with 1GB Patriot RAM that previously killed boot sector of WIN XP because of reboot in 2-10 secs. Now planing to add more memory http://m79.lv/product/single/group/2..._CAS_3__RETAIL ..
Thanks for help!