my presario r3000 won`t stay powered up

my r3000 won`t stay powered up either by the battery or ac power.
sometimes it works for 5 mins sometimes 30 sec.
but it works great for the 5 min and then just shuts off.
i took it to a local shop and they told me it was the motherboard and said that they would wave the fee to check it out if i just let them keep it.
the motherboard in there was already replaced before under warranty for a different problem.
it seems to be a power issue. can a power issue in the motherboard be repaired without replacing the motherboard?

 

Wrong section. Should be in HP.

 

Moved to HP/Compaq.

Usually that behavior suggests overheating caused by a clogged heatsink, which is relatively easy to fix on that series. Battery would last longer than plugged-in because the BIOS restricts the maximum clock speed a bit while on battery to conserve power. Anyhow, get the service manual from hp.com (downloads page for your notebook, Manuals link on the left-hand side) and follow the instructions for changing the CPU just far enough to get to the heatsink. Don't forget to remove the battery first.