Sandra:

If your computer has been off for a few hours (or overnight), can you then start up, and quickly load all the programs without a problem? Or does the problem ONLY happen after it's been on for a while?

When is the last time you removed the cover, and blew/vacuumed out the inside?

The blades of the cooling fans get "crud" on the them that impairs the fan's efficiency. Once a month, I power down, turn off the rear switch, unplug the power cord, remove the cover,and clean everything thoroughly. I use canned air in conjunction with a vacuum cleaner (being careful to use short intermittent blasts from the can so as not to freeze anything). Ensure that the cooling fins of the CPU are clean - lint is a very good conductor.

I'm with Mac - I would first rule out over-heating. It's the easiest first step.

On my music computer, I upgraded from the original to a high quality CPU cooling.

Glenn