Just before Christmas, I bought my daughter a Rise at RS for an incredible 20 bucks. That phone is essentially the same as an Event but with a slide-out keyboard. I was, of course, decided to test and set up the phone prior to activating it on Virgin when she got the phone on Christmas day.
I didn't realize it at the time but I screwed up by inadvertently pulling off the entire cover/label from the battery. It seemed at first to be fine but then I started seeing that the thing was just dying or shutting itself off at various times. Worse, it would not turn back on until you removed/replaced the battery. I finally decided to give Kyocera a call and they told me to call Virgin Mobile.
So I did, described my problem, and the tech agreed to send me a new battery. I had not mentioned the thing about the label because, frankly, it hadn't occurred to me that might be a problem. So, the short of it is, that solved the problem and the phone has worked perfectly ever since.
Hope this helps someone resolve their problems with, really I guess, any Virgin Mobile phone.