Computer Crashes itself in every 5-10 mins

It’s a biostar pc with XP pro. It runs well while it’s connected in the LAN.There is no problem in the office but in the home while I have dynamic ip address( cable), it just crashes itself in evey 5-10 mins. I loose power and and comes back on.Mostly hapens while I am in the internet but when I am on the internet in the LAN environment it just runs well in everywhere. I was trying to format the disk before. I notice ntldr missing then I got rid of it.But now it just doesn’t work. Some told me it’s porblem with the motherboard. I installed new motherboard but still same nature. It’s my boss computer , please let me make her happy.Advice please