Horizontal bars moving on display due to interference.

Cause:

Electrical power interference from nearby electrical appliances that are not well shielded (such as, laser printers, fluorescent lights, or major conduits) is interfering with the computer display.

Solution:

Move display (and computer) to a location with less electrical interference.
NOTE: The same interference causing problems with display may begin to affect the hard drive also. Avoid work areas with high power interference problems.