Computer locks up randomly, Help Me!

My older computer has these specs:
1.2ghz amd athlon thunderbird
Motherboard (thats the brand) 663as ultra/ 663as pro
1x spectek 256 sdram 133mhz; 1x dell 128 sdram 100mhz
ati radeon 7000
27gb hdd
windows98

Recently, a strange problem started occuring on my computer. Usually just after startup, (10 mins or before), the computer will completely freeze. No mouse movement or keyboard reaction at all. It seems like completely random times that it happens. I used to be able to play battlefield with no problem, besides sucky graphics, but now it completely locks when i try to play a game. Some days, every thing is fine until I try to play a game or render a video project. Other days, it freezes up right after windows loads or even before that, (even at the blue scan disk screen). I usually have to shut down with the power button and let it sit a few minutes. If I press the reset button, it usually shuts off the monitor and then doesn't turn it back on, but the computer keeps running without responding. I have asked many many people about the problem and have come nowhere near an answer. Please help!
Thanks

Welcome to TechSpot
Take either piece of RAM out and all should be well.
You should never mix different RAM-types (133 and 100 MHz in your case), and when you use more than 1 stick of RAM, they should preferably all be from the same manufacturer.

2 different speeds pull the whole lot down to the speed of the slowest stick, which may also not agree with your BIOS-settings.

Look for the likes of Crucial, Kingston, PNY

I tried both pieces of memory. Leaving in the 100mhz stick, there was some sort of error about the system registry. Leaving in the 133mhz stick, the problem was still there. Thanks for trying though. I'm wondering if it might have something to do with the agp or chipset, because sometimes when I press the reset button after a lock up, the monitor says frequency over-range. Also, the problem seems to happen when graphics/video proccesses are running.