Screen goes black

I tried vacuuming all the vents but it didn't help. I hve taken it in to our IT people. It seems the video card is shot--apparently a common problem with this vintage iMac. It's too old for parts to be available, so the "cure" is to buy a new machine.

Thank you all for your suggestions.

I don't think I have these third-party apps. The problem syarted a couple of days ago, when the screen went black after about half an hour of use. Since then the "live" time has got progressively shorter--now a few seconds. My guess was that it was a temperature sensor malfunctioning. In any event it looks like a hardware isssue.

This didn't seem to help. The screen is now going black/blank a few seconds after the computer is turned, which doesn't leave much time to try things.