That machine could be 7 years old.
When's the last time you cleaned it?
If the monitor is plugged into a video card plug it into the mobo and test.
If it shows the same problem test the monitor and cable on another pc.
Video Card, Monitor or other Failure?
My HP Pavillion s5-1060PC Screen Goes Black 10-15 minutes after boot and stays black for 30 seconds or so, when it comes back up a box displaying;
Auto Switch Source -On
Default Source-DVI etc. etc...
Anyone else have this problem and what was the fix?