What is probably happening

What to do

Possibly a mismatch between motherboard clock jumpers and CPU speed;

OR

It's overheating.

If your CPU runs at 1200 MHz, make sure you have your motherboard clock jumpers set the same way--not at 1300 MHz;

OR

Open the case, and check for excess dust. Inspect the fan to make sure it is secure and working properly. Also make sure the machine has good air circulation around it.

OR

You are running programs that don't play nice. In that case, reboot the computer. Remove all programs from your start menu. Use Task Manager to shut off as many programs as you can. Open the various programs you have loaded, and disable "run at startup."

Then, reboot. If your problem disappears, enable one program per day to run at startup, until the problem comes back. Then, you'll know which program(s) to contact the manufacturer about to get the problem resolved. In some cases, you'll need to just stop using the program--especially true of shareware. In some cases, you are running a program that you never loaded--these come via e-mail, Web, etc. You can probably uninstall these via the Control Panel.