"Bouncing"/flickering image with monitor

[Syzich]

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.