I assume you are running SIP code. If so go take a look at this website:
http://www.voip-info.org/wiki/view/Asterisk+phone+cisco+79x1+xml+configuration+files+for+SIPTake a look at what version of firmware you are running as compared to the ones they discuss. Could be buggy firmware. I have 2 7961G running at the house on asterisk with SIP but I have never seen your problem. The 7961 and 7941 use the same firmware code.