Cisco 7940 reboots

I have recently starting playing with asterisk. I have installed Asterisk@Home 1.5. I have not changed any of the base configuration files. I configured 2 extentions 200 and 201 using the GUI. Both of these phones are Cisco 7940s with SIP Firmware installed (P003-07-2-00). Everything seems to work fine except when pressing digits on the keypad of the phones they will randomly lockup and/or reboot the phone. Has anyone else seen or know of a resolution to this problem?

Thanks in advance