First, try this:
http://docs.info.apple.com/article.html?artnum=303319
If you are comfortable opening your MacBook Pro, check the RAM and verify it is seated correctly. Try moving the RAM, or taking a chip out.
Another theory is that it's heating up for some reason, and the fans are not kicking in, hence the shutdown. This could be caused if they forgot to connect the heat sensor back to the new logic board. You may have to bring it in to fix this, or open your MacBook Pro completely (void warranty?)
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Power to Burn.
At speeds of up to 733MHz,
The most powerful Mac in history
burns CDs, burns DVDs, and
burns Pentiums
- apple website, oct 4, 1999. advertisement for the powermac g4