Crashing, freezing, glitching despite good specs

So, I go onto SL and sometimes (the time between it's crashes vary) It'll sometimes work for five minutes (or ten) before the screen freezes, the monitor clicks (and turns black for up to ten seconds) and then it won't resume the game - and I usually have to close the program completely before re-opening it.
I've disabled sound and reduced draw distance to 64 metres and disabled most other things to stop it from over-loading. Specs:

Windows: Windows XP5.1 (Build 2600) Service Pack 2
Memory (RAM): 512 MB
CPU: AMD Athlon(TM) XP 2500+
CPU Speed: 1831.1 MHz
Sound card: SoundMAX Digital Audio
Display Adapters: ATI Radeon 9550 / X1050 Series | ATI Radeon 9550 / X1050 Series Secondary | NetMeeting driver | RDPDD Chained DD
Screen Resolution: 1024 X 768 - 32 bit
Battery: No Battery
Motherboard: ASUSTeK Computer INC. A7V8X-X

I used to run SL on a machine with similar specs, except that instead of an ATI card, I had a GeForce 6600GT and 2GB of system RAM. I suspect that you may need to uninstall, then update your video driver for the ATI card. It would also not hurt to have at least 1GB of system RAM (minimum) for SL to run well.

Yeah. I know LL's system requirements still say 512, and I sometimes run on a laptop with 512, which is stable but very unpleasant. I wouldn't be at all surprised if this was your problem, tho. More memory is really not that expensive ... try it!

The ATI cards of that variety are plagued with the symptoms you are having, it all stopped when I switched to Nvidia 6600 GT. ATI and OpenGL did not get along at all. Modern ATI cards and drivers are a lot better now although if you were using the computer mainly for SL, I'd still go with Nvidia.