Yellow exclamation point in white triangle error

[harbinbishop]

My Thunderbolt is stuck on a screen with a yellow exclamation point inside a white tiangle after it boots up and is working for a minute or two. I've managed to get it to a "reboot screen" somehow (pressed home button, power, volume buttons in some sort of combination). I've tried the option "reboot system now" and came right back to the yellow exclamation point. The phone was "rooted" by a kid a few months ago, if that makes any difference. The info at the bottom of the "reboot screen" says:
Revolutionary CWM v4.0.1.4
Finding update package...
Opening update package...
Verifying update package...
E:failed to veriry whole-file signature
E:signature verification failed
Installation aborted

Thanks!