game freezes unpredictably

I'm playing Civ 4 BTS on Windows 7 64-bit Home Premium Service Pack 1. My computer is an Acer Aspire 5251 with 8 gig of installed memory. For the most part the game is working just fine. But without warning and completely unpredictably after an hour or more of play the game will simply freeze and the computer becomes unresponsive. Sometimes I can wait it out, and after 10 minutes or more the game will resume just as if nothing had happened. Other times I have waited, and given up waiting after nearly an hour. In such instances I feel I have no other option but to shut down the computer and give it a hard boot. When the game freezes, the only sign of life is an irregular series of buzzing sounds, all of the same frequency but quite varied in volume and timbre. Can you suggest a cause for my problem, and or a possible solution? The problem is completely unpredictable. Sometimes I can play for hours at a time with no problem. Other times the game freezes after only an hour or so.

Might be a hardware issue. How hot does the system get while ingame?

One possible cause might be that your memory is just slightly unreliable. I'd run a thorough test with https://en.wikipedia.org/wiki/Memtest86

The CPU temperature rises to a maximum of 95.8 degrees C. I ran Memtest86+, but the computer shut down during the third pass. I ran it again, and the same thing happened.

Dang, that sounds like a serious overheating issue

Maybe it's dusty? See if you can clean the fans for instance; that might help.