Computer help forum: pc monitor seems to go in sleep mode and will not wake up

my simple pc with XP, started going into what appears to be sleep mode a few months back. This was happening very sporaticly like one a month or so. When it happens, the monitor would click off, the light on the front of the monitor will flash of and on slowly. The computer will not wake up with any touch of the key board. The only way to get the computer to respond is to turn off the power strip and restart the machine. This is now happening all the time. Sometimes the pc will run for a few days, some times only a few minutes before the monitor clicks off to sleep.
I am not sure if this is a hardward, software, or some other problem.
Some variables:
1. I re-built the machine last summer. I do not know if it was happening prior to this. All same software. The hard drive crashed giving me the oppertunity to upgrade the mother board. I canged out the video card at the same time. i do not know if the machine was doing this prior to the re-build ot not.
2. We have a logitech mouse and keyboard.
3. We upgraded to DSL from dial up in the last few weeks.
4. The monitor is seven or eight years old.

Things I tried:
1. Crtl, Alt, Del. Does nothing.
2. Went into bios and xp power settings to turn all on and turn all off. same thing either way.
3. Removed the wireless keyboard and mouse and put the old ps2 devices back. Same thing happens.
4. Unplugged the monitor from the computer and power when it happens. Plug back in. Same thing happens.
5. Deleted the niverda video driver and had XP select a generic driver on start up. Same thing happens.

??????????
Things I was thinking I might try:
1. Find an old video card and swap. This wouold tell me if there is a problem with the card. Not sure how a video card would break and act Not sure if this is of any value or not???
2. Find another monitor and swap. This would tell me if there is a problem with the monitor. Not sure how a monitor would break and act like this.
????????????????????