bios clock loosing time

I have a client whos OS is win XP, computer IBM netvista Zpro. his bios clock looses 5 hrs ove a 48 hr. i have changed bios batt twice, changed mother board, downloaded nsf chip drivers from microsoft, checked get time from internet bullet in XP clock setup called IBM helpdesk they say that they don’t know what else to do. any suggestions woul be much appreciated dan