Wed,
Your cable combination is probably causing your problem.
- high quality extensions can run as long as about 30 ft (total) for 1024x768@60Hz resolution. Higher resolution settings will reduce useable length. Several companies sell amplifiers/repeaters for even longer lengths (expensive). Try a lower resolution setting or a shorter cable.
- bad shielding in the cable(s) or especially at the connector(s) are prone to interference problems (ghosting, noise, etc.). Try another cable from another manufacturer. Try a different length.
- placement of the cable(s)/connectors can introduce noise. Your monitor, PC, other cables and nearby electronics can generate a lot of 'noise that can be picked-up by your cables (including itself). Move the cables away from other electronics/cables.
Dell