If its a CRT monitor and it happens often then yes, its probably dying. That happened before my CRT montor died atleast. When I think about it, i've seen many monitors die like this.
Fortunately CRT monitors are rediciously inexpensive today so it's not that bad. Its worse when pixels die on LCD-monitors =)
Its possible it could be a bad power cable or VGA cable, too. I'd swap those out first if you have spares handy to see if there's any change before running down to the store and buying a new monitor...
I got the video card at end of March. I noticed it acting up begining of May. Also, it only acts up when I'm on the computer for a long period of time, after like 5 hours or so. Never used to do that.
I suspect cable or monitor. Of course you might not be able to replace the cable on your monitor. It depends upon who built it so your probably looking at needing a new monitor.