HP Pavilion Shutting Down No Signal To Monitor?

I have an HP Pavilion a6300f with an Nvidia GeForce 8800 GTS video card and an HP S2331 23" monitor. The tower is about 7 years old. A few weeks ago I began coming in and seeing "No Signal To Monitor" as a screen saver on the monitor and no amount of moving the mouse or tapping keys would wake it up. I'd have to reboot the system with the power button. Now, in addition to that I come in frequently and the system has rebooted for no reason because it's on my password screen. Sometimes the power button will be yellow as if the machine is asleep but nothing will wake it up and again I have to power it down and power it up manually. Sometimes when I do it will say "Resuming Windows" on the monitor, the blue boot up screen will come on but when it actually starts to boot I get the "No Signal To Monitor" all the sudden. This used to be infrequently. Now it's happening if I leave the computer running without use for more than a few minutes. I'm thinking I need a new computer but if this is an issue someone is familiar with and you know of a solution it would be greatly appreciated.