Running hot with Ubuntu

ASUS P5WD2 motherboard, Intel Pentium 3.0GHz dual-core processor, 1GB RAM, Sapphire ATI Radeon X800GTO video card, Ubuntu 6.06.1, just updated the kernel and the fglrx video drivers to the newest available by Synaptic updater. (I installed the video drivers via easyubuntu.)

Often (but not always) when I boot Linux, my computer starts screaming at me... a high-pitched squeal that I presume has to do with the processor running too hot. When I run "top" I see that mail-notification has started and is trying to connect, but since I am on a dialup account it gets frustrated and takes 100% of the CPU. I kill that process and it stops screaming.

(It does the same thing whenever a program hits the processor hard, and stops when the process finishes hogginf the CPU.)

When I try to play a DVD, it starts screaming immediately. Odd, though, that when I ran "top" it looked like the processor was fine.

One thing that always stops the screaming is switching to a full-screen shell session, so I'm wondering if it could be the video card or the fglrx driver that's causing the problem. All of the stuff I've found via Google pertains to laptops; it looks like desktop chips don't have the throttling capabilities found in laptop chips. (Or do they, and I'm overlooking this somehow?)

Note that I never hear the noise under either eComStation (OS-2) or the occasional Windows XP boot. I heard it once under XP when I was trying a buggy piece of software that locked it up, but it didn't happen right away... it took a minute to heat up.

As I'm new to Linux, I'm not sure what other information a knowledgeable user might need to help me track this down. I *really* want to move to Ubuntu (or at least some distro) but the noise is keeping me from doing so.

I'd appreciate any guidance you can offer. Thank you.