Help phone will only boot when connected to power?

[aadcrasher]

so, im really confused right now. ive been having issues, and i thought i had it worked out, then i tried rebooting my phone, and it kept looping the white htc evo screen, so i plugged it into the computer to do some adb/fastboot stuff, and it booted. ive done this multiple times to confirm. it will also boot when connected to a wall power supply. once i see the animation begin, i can unplug it and use it normally, except my usb mass storage doesnt work, and the phone doesnt either, but those could be unrelated. any ideas?

thanks in advance
-Alex