Actually it sounds like bad system ram to me. Your memory is fine at 100 MHz but is having problems at 133 MHz. Try downloading and running memtest86 (
http://www.memtest86.com). This program will make a bootable floppy (or CD) and will take about 30 - 60 minutes to complete a test cycle with 256 - 512 MB of system memory. (Run it with the FSB set at 133 MHz.) If you see any errors, run the test again and if the error is again present you will want to pull memory sticks and try to track down which one is the culprit.
If it's not that, the CPU may be overheating or the power supply is very borderline and the extra CPU clockspeed (and corresponding rise in power consumption) is just pushing it over the edge. The 9500 PRO draws a lot more power than a GeForce 2 does. You can try to track down a heat problem by opening the side of the case (to improve cooling) and trying to play AH again. A power supply issue can be a lot trickier to track down, as the system will usually reboot without any warning if the powersupply overheats or glitches.
You might also check (if possible) in the bios that the AGP bus is running at 66 MHz. If the FSB is overclocked on a VIA chipset board, the AGP bus also rises in clockspeed. Once it gets to an even divisor of 66 MHz it will be corrected. (Meaning if it really is at 133 MHz, the AGP bus should be at 66 MHz, but if it's accidentally been set to say 120 MHz it may be different.) If all else fails, and you have the option, try setting the AGP transfer rate to 2x.
One last thing that may be helpful is to check for errors in the system log. Right click "My computer" on the desktop and pick manage. System tools -> event viewer. Look for any errors under system and/or application. Typically a message indicating that the system has rebooted from a bugcheck may be helpful in narrowing down a software issue. (Unfortunately this sounds like a hardware related problem though...)
