Probably a bad LMB, at least that is what it was when mine was doing it. You can call E* and they will send a tech out to change it for $29 if you have the DHPP, or you can buy one and replace it yourself, just make sure you get the right one.
EDIT: I am guessing you have DPP equipment? I only had this problem after I got a DPP LMB and almost right after I got it.