I've had this issue with two HTC phones. Mainly when opening and closing the keyboard, then it just happened if it moved too quickly, vibrated or was looked at wrong. I heard the issue was the battery among others. I was starting to think it was something in the power button. One HTC got demolished out of anger, the next one (both used) I decided not to destroy.

I took the battery connection advice, opened the phone and saw there was a slight wiggle to it. So I took electrical tape, put several neat strips along the top ridge so that it needs a bit of pressure to push battery in. When that was done I had no wiggle to the battery anymore. For extra security I put little strips around the battery (small ones as wide as a pinky finger, just in 4 places to make sure it could still ventilate but held the battery more secure). After that I put the cover back on. I used a little tape holding the cover too, but my cover is wore out and barely held in place, you probably won't have to.

The outcome: it has been about three days since I have done this rig and so far so good. I can open it, close it, keep it in my pocket and all the other things that are used to transport a cell phone and it has not turned off on its own yet. Several calls, several messages. I can imagine on an older phone, the teeth that connect to the battery may not be as true as they were and the battery gold etches away a little, just enough to cause a momentarily (secondtarily?) loss of juice, hence turning off. If it doesn't bounce into place again, of course it doesn't want to turn on. I know electrical tape is a rig and by no means is a perfect looking fix, but so far it has worked wonders for me and took only a minute or two, just make sure the tape is straight, on the top of the battery holding it in place. 3 days and I'm happy, now whether tomorrow I'll find out if my rig is no longer working I may take back these words, but so far I am happy.

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