"Bouncing"/flickering image with monitor

I'm not sure exactly how long it's been doing this now, but for the past month or so, I've noticed the image on my monitor will "bounce" once in a while. It looks as if the entire image on the monitor shrinks slightly, then goes back to it's original size. It's very brief and barely noticeable. When I first remember seeing it, I thought it was just because I was tired and my eyes were a little strained(I'm still not 100% sure it's my eyes or the comp, I do spend lots of time on the computer, though I'm in a well lit room and I'm not all that close to the monitor. I haven't noticed this happening on my laptop, though.). It's kind of hard to explain. Is this a sign my video card or monitor is about to die? They're both 5 years old now. I have an nVidia GeForce 4 MX420 video card(driver version 6.176), an E772c Dell Monitor and Windows XP Home SP2, if that helps.

It sounds like the problem could be with the high voltage power supply for the CRT. It is possible that enough dust has accumulated inside to cause problems, or it could be a component failing. If you decide to remove the cover to blow the dust out, be careful. Some of the internal components can give you a nasty shock even when the monitor has been unplugged for quite a while.

Make sure the monitor cable isn't routed too close to any transformer blocks or anything like that. Sometimes they can pick up transient signals.

I got a Wii shortly before I first noticed this "bouncing" effect on my monitor. I have the Wii sitting on my desk and it's power brick was sitting on my computer tower, near the back, close the the monitor cable. I moved the power brick, so maybe that will take care of the "bouncing". If that doesn't take care of it, I'll remove the monitor's cover and take care of whatever dust may be in there.

Thanks for the replies :) .