hello guy..i have some problem here..

my hp always hang when i receive or making call..it will hang after 15 minute conversation..after that the screen will go blank..i cannot on the phone unless i take off the batery and restart again..firstly i think its batery problem, after buy a new battery its happen again..i had format my hp and my memory card..but this problem still happen..i also had reinstall my new firmware but still happen.. what should i do??? i think it is because my new firmware after i upgrade it..my firmware is v5.0719.0.2

please anyone help me...

my n80 was originally i.e version..but when i upgrade my firmware, all the yahoo software thing are gone.. can i upgrade my phone back to i.e version? and how to to it? please anyone help me....