HELP!! Rainbow screen!! will only boot with usb plugged in.. anyone heard of this??!

[zerodegre]

My phone is Alltel.. was working fine.... then out of the blue it went to rainbow screen... now everytime i soft reset it goes straight to the rainbow screen and ways "serial" and will not boot...!! However,,, this is where it gets strange.. if i plug the usb cable in for a split second it will soft reset and boot fine! and work fine!! CRAZY.. it does not have to be active synced or anything.. just if it is pluggrd in even for a split second it boots... once it starts booting i can remove the usb cord and its fine... but i cant soft reset without it being plugged in.,. HELP

PLEASE