Problem with GeForce 8600GT. help!

Hi I've been using a XFX GeForce 8600GT 256mb on my comp for 7 months and i had no problem with it till the past week. Sometimes when I boot my computer, the screen does not display with my monitor plugged into 8600GT but when i replug it into the motherboard video card, it works. I tried cleaning out and reinserting my 8600GT into my PCI Express 16 slot a number of times but still does not solve the problem. I'm using my "motherboard video card" right now. However, the weird thing is sometiems when i reboot my comptuer, screen goes out with the motherboard video card and when i plug it back into 8600GT, it shows a display. So, basically both of the video cards have been going on and off and i have been fighting with plugging and unplugging with the monitor plug. Is this my video card problem? or my motherboard problem? When the screen displays, the video card works fine.But it's just on and off. Any comments?