Do you have another computer that you can test the monitor on, or another monitor you can test the computer on? I would suggest trying that so you can narrow it down to one or the other (or neither, if it's a problem with the cable you're connecting with.

Also, you said you moved it closer to an AC vent, but it might be worth opening your desktop and cleaning out any dust inside - that can easily cause it to overheat if you haven't cleaned it in a while.

I also found this HP forum post (also for an HP Pavillion) that suggests going through your BIOS and resetting your video settings. It's possible that a Windows Update may have messed up something with what port of the graphics card it works with, so you might need to reset the video settings in the BIOS to that.