Hi Brian,
First, how much free space on your HD?
Have you inspected the interior for dust buildup?
Have you booted off the Install Disk by holding the d key down at boot-up, (not the c key), and run the extended Apple HW Test? Some Install disks require holding the Option key at bootup to select the AHT. Some Macs came with separate AHT CDs.
If that turns out OK...
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