Desktop Repaired With New Hardware

Gets be busy, as with many tabs, two browsers, and several other programs running, when the monitor would go black. Not gray, but black as if it had no power. It would last usually around 15 seconds, occasionally till I did a cold boot. When it came back from a 15 second episode, sometimes colors were strange and or parts of the screen remained black. It acted like my ancient '98 had when out of resources. But that was rare --usually it just came back and we kept going. I contacted HP and they tried some diagnostics (as in, they controlled my cursor), with no trouble apparent. The told me I'd have to mail it to them. Knowing they usually format the hard-drive, and seeing this as a lot of trouble, I insisted they send a tech.