The symptoms you are describing could be caused by either hardware or software problems. Attempt to eliminate software first, then troubleshoot your hardware.
Try these steps in the order presented. If a step fails to correct the problem, proceed to the next one.
1. Restart your computer. Many times simply restarting fixes the problem.
2. Restart and turn off all extensions. If the problem goes away, troubleshoot your extensions.
3. Shut down your computer, but leave the power supply to the computer turned on. Switch off any SCSI devices and completely disconnect all Apple Desktop Bus (ADB) cables and devices attached to your computer. Wait at least 10 seconds, reattach the ADB cables and devices, and restart your computer. Warning: Do not disconnect ADB devices while the computer is turned on.
4. Reset the Parameter RAM (PRAM) by holding down the Command-Option-P-R keys at startup.
5. Replace the ADB cables and devices.