P4S5A and RAM problem

Hi everyone!

New one here with question to a problem. Originally when I bought P4S5A mainboard with my PC it came with 256Mb of RAM running at 266MHz. About two years ago I upgraded with Kingston KVR 333X64C25/1G (1Gb running at 333MHz). NOTE: I was using ONLY 1Gb RAM, not 256Mb anymore at all. Everything was fine until few days ago. PC stooped working.

After initial panic that maybe HD died on me and replacing one by one all the components I tried removing also 1Gb RAM and replacing it with old 256Mb RAM and things are back to normal with it. Thinking that found the problem went out and bought new 1Gb same model of RAM but today 100$ cheaper then 2 years ago. New one is also NOT working. Tried both slots and it is not working. Now I again placed 256Mb back in and it is working.

Since old RAM running on 266MHz is working would it be problem that for some reason motherboard is not supporting anymore 333MHz? Now I found out at the store that Kingston would replace the old one and if another guess of mine makes sense I will be stuck with 2 X 1Gb of RAM that I can't use and will have to buy new running at 266MHz. (store where I bought RAM has no return policy on it)

Does anyone came across similar problem or have any idea what should I try next?

Thank you all in advance!

Kris