Dimension 8100 not booting up and has amber light on power button. Please HELP?

I have a dell dimension 8100 that has been powerleaped from a 1.7 to a 2.5.  I recently upgraded the RAM from 256 PC 800 RDRam to 512.  I also have upgraded from a GEForce 2 Ultra to a GEForce 4 Ti4600 quite a while back.  Everything has been working great until the last week or so and I am at a loss here.  I used to leave my computer on all the time 24/7 with reboots here and there when I needed to.  Well, after I lost the fan on my Video card I started to turn it off.  After about six months of this it decided that it doesn't like to boot up after a night or two of being off.  What happens is when I push the power button you can hear it power on but the monitor says there is no signal being sent from the computer.  The other weird thing is that the power button light is amber instead of green.  I turned off the computer and unplugged all power from it and tried again.  This time it booted up.  The last time it happened, unplugging it didn't even work so I pulled the case apart and cleaned it out.  I yanked the Video card out of the AGP slot and reseated it and turned it on and presto it fired right up.  What gets me is when I leave it on and have to reboot it, everything works fine.  It only acts up when I leave it off over night or longer and then start it.  Could this be the battery on the motherboard?  My clock doesn't seem to be losing time in windows or anything.  If anybody has any ideas please help.