|The codec has nothing to do with your problem.|
The most likely cause is a loose monitor cable connector. If it's not loose it's probably a broken wire in the connector. You can test that by watching the screen and wiggling the monitor cable. If the red appears and disappears, it's a broken connection.
If it's an LCD monitor, you can probably replace the cable.
If it's a CRT monitor, it would be pretty expensive to repair. Probably best to replace it with a LCD.