Underclocking makes my computer unstable

[Clopedion]

I've got a 1.2 GHz Athlon and an A7A266 motherboard, and I've been trying underclocking my CPU to 900 MHz by changing the bus frequencies to 100/100/33. However, often on Windows startup and occasionally other times, the computer will freeze and a blue line will appear across the middle of the screen(orange when it's on a white background). This doesn't occur when it's at 1.2 GHz, and Linux seems to run fine.

I've tried reinstalling Windows(at 1.2 GHz; the installation crashed at 900 MHz), which changed the position and color of the line; it used to be green and about a third of the way down.

Does anyone have any ideas?

 

[crisp82]

I found that when I underclocked my video card that my system slowed running desktop, and became unstable.

Why would you want to underclock a 1.2ghz cpu to 900 anyway? Can't you're motherboard handle it?

 

[Clopedion]

My case isn't very well ventilated, and over the summer my computer will be in a fairly hot place. I don't really use the speed that much anyway; I used it for some heavy math computing over the school year, but not much since then; I play games, but nothing really recent.

I run Prime95, but that doesn't seem to care very much; even at 600 MHz, it runs at the same speed, so evidently there's some other sort of bottleneck.

I do underclock my video card(the fan broke and I haven't fixed it yet), but I didn't run into any problems with that before; maybe I'll get around to fixing the fan and see if that changes anything.