This is a really strange issue that I've finally been able to pinpoint. Here's what happens: I turn on my RPi, and in a few minutes, all USB devices shut down, and if I have Ethernet plugged in, it loses connection. Now you may think my USB devices are drawing too much power, but they were in a verified powered USB hub. When I tried without a hub and just a mouse, the same thing happened. So then I tried without any USB device, just Ethernet, and still happened. After this, I think I may not be giving it enough power (somehow?), and I check. I am using a micro USB adapter with an output of 850 mA. (Also tried 600 mA), so now I'm at a loss. I'm willing to try anything.