Monitor going wacky with extension cable attached

I have (this is on my mom's rig actually) a ViewSonic VA520 flatpanel monitor. I recently had to add an extension cable to get the monitor square with the keyboard (monitor was previously closer to the tower on the other end of the desk than the keyboard tray...ugh). After adding the extension cable, I'll get some weird intermittent yellow/green tint shifts that seem to sort themselves out after about 10 minutes, only to reoccur if the monitor is unplugged from the comp for any reason.

Could the problem be the monitor connections or the extension cable? I do recall it happening before I put the new cable on...

move the cables around, it's possible emag from the power cable is interfering with the VGA signal.