Gmod: Memory could not be "read"

I honestly have no idea why it does this, but whenever I join a server, the game freezes for a second with repeating sound, then noise stops, then "dung". Game crash. It has just done this 3 times and has been doing so on and off for the past 3 months or so. I have verified game cache files on steam, defragged disc, I am pretty sure I do not have fault RAM since I just got rid of one faulty RAM stick. Why does it do this? Hardware just not compatable/too old? (One of the times it crashed, as soon as I went to desktop I got a BSOD saying a ati file caused the bsod.)
SP3, XP Home Edition 2002, Pentium 4 CPU 2.40 gh/z, 3 gig RAM.
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It's probably not your graphics card, but out of interest what are you using? As for old hardware, that certainly could be the problem. Your CPU should just about be able to handle it, but only at low resolutions. As for faulty RAM, are you using three separate 1 Gig modules, or a 1 Gig and a 2 Gig? I'm assuming that you simply replaced one stick with another, but in either case you will need to check the older of the two modules. Also, trying to run three modules is much better than trying to run two unmatched modules, but both are unstable compared to running two matched modules. (So something like 2x2 Gig or 2 x 1 Gig). I could have a look at the minidump for you, but I know that other people on this forum will be able to tell you more about it that I will.

Oh whoops, ATI Radeon HD 2600 Series AGP. I have 3 seperate things of 1 gig RAM (I had a fourth but it ended up being faulty)

Hmmm, okay. I assume you ran a memtest to find out that the fourth module was faulty? If so, try running it again just to be totally sure that none of the others are having problems. If they all pass, what I suggest you do is take out the third stick of RAM so that you're just left with two side by side. Try Gmod again and see if it pops up with an error. If it does then we can sort of assume it's not going to be a problem with the RAM setup , so you can just put the third stick back. As for the HD 2600, I'm not entirely convinced that it's going to be enough to run Gmod. Obviously your rig meets the requirements, but they are a tad misleading. A friend of mine was trying to run Gmod on a Intel Core Duo with an Intel 945 graphics chip. I think his game started but crashed as soon as he tried to do anything. Obviously he didn't have a dedicated graphics card, but the issue was with a complete lack of computing power. Sooo, what I suggest is that you try running the game in DirectX 8.1 mode. Obviously it's not going to look as nice, but it should mean your GPU wont be over stretched. To force Gmod into DX 8.1 mode you need to; 1. Open up your Steam games tab. 2. Right click on Gmod and go to Properties. 3.Once in Properties click Set Launch Options and type in "-dxlevel 81 -w 800". Obviously don't include the quotation marks. What this should do, if all goes well, is force Gmod into DX 8.1 and into a 800x600 resolution. If that doesn't work then I suggest waiting for a reply by the WTT experts. They may have some suggestions regarding diagnostics to figure out if there is anything wrong with your hardware/