Vaio Z - Interference on external display

Hi, Has anyone experienced electrical interference on an external display?

I have a customised Vaio Z4 and have an issue that an external display connected via VGA suffers from slight electrical interference-- a wavy horizontal distortion that is most noticeable on dark areas of the screen.

Im wonder whether to send the machine back to the factory for a check up. I have already had a replacement PSU supplied by Sony without resolving the issue.

The following methods reduce the amount of interference:

- Change the power plan to one that disables devices, such as: DVD drive, modem, etc. The more devices disabled the greater the effect;
- Change to ‘stamina’ mode with the Intel power saving options enabled.

The following methods eliminate the interference:

- Using an older Vaio power supply that has a lower amp rating-- 19.5V/4.1A instead of the regular 19.5V/4.7A;
- Connecting the external display to a power socket in another room using an extension cable. Using a different power socket in the same room does not work;
- Connect the external display using HDMI/DVI-D instead of VGA/VGA.

Thanks;