Ok I had a similar problem with my Roommates computer, it was an older P4 running on a MSI MoBO with an early 2/4x AGP slot. I put in a MSI Radeon 9250 and started having problems. I recently built a new system and put that video card, just to test it, in my "new" system and found that it didn't spaz out anymore. I also had been running the MSI PC Alert program to keep track of the Processor temp, and on both systems the CPU and System Temp was well within acceptable ranges.

Here is what I came to think. Although some MoBo's have 4x/8x support, not all chipsets are created equal. While some can support running at the limits (most newer cards push the limits of the architecture at all points) others can't.

About the only thing that I can honestly suggest here is to make sure your running the latest BIOS for the MoBO in question and the latest drivers for your vid card. There is another option and that is to change the AGP controller driver from the stock PCI-AGP bridge to PCI-PCI in the Windows Hardware Manager. In some cases this will fix the problem, while this will impact your overall FPS rate and also cause you to lose some of the AGP features it will cause some crashing issues to dissapper.

Before doing the afformentioned "fix" I would like to once again suggest making sure your running the latest BIOS flash for your MoBo and the latest drivers for your vidcard and chipset, seeing as how this could very well fix the problem your having. Also remember that the Sempron is AMD's version of the Celeron, and neither chip is really ment to be in systems that are for any type of gaming, they are both underpowered versions of the main chip lines. They usually have less cache then their full powered main line siblings and lower clock rates, generally.