NOTE 1: Remove all external add ons: extra memory, hard drive, external disk drive printer, modem. This elliminates them as a source of the problem. If computer works normally when all add ons have been removed, start adding devices on, one at a time, until the problem comes back. Then determine which is at fault, the device or the computer.
NOTE 2: Substitute a known good disk drive and power supply so as to to eliminate them as the source of the problem