Registration already pending with SIP

I’m not sure why this would be happening, but after my server with asterisk has been running for awhile, working perfectly, it stops working. What I find when I log in to it is this:

NOTICE[2215]: chan_sip.c:13277 transmit_register: Strange, trying to register when registration already pending

I can’t figure out why this would happen suddenly. After I restart the server it works again, but the problem is if I’m on the road, I can’t restart it in a timely manner.

Any thoughts?