I found that if you remove the SIM card, reseat it in its frame, then reinsert it, it seems to solve the problem.
Turn off the phone.
I got a paperclip and popped out the sim card by putting the end of the paperclip in the tiny hole on the side of the phone (its on the opposite side of the volume controls). This will pop out the sim card.
Take the sim card out of the casing, blow on it for good luck, then place it back in the casing.
Push the casing back into the phone until it snaps into place.
Turn the phone back on.
It's been 24 hours and haven't had any issues since. Hope this saves you from standing in line at the Apple store....