What's the make/model of the laptop? Does it happen when you aren't surfing or connected to the internet? Does your laptop get hotter than it used to? Is it hotter in the room where you are using the laptop than before?
If it's heat, you might first just try putting a few cd/dvd cases under the laptop just on the right and left sides so there is a good air "gap" between the case and the table. That should lower the temperature of the laptop. If it lasts longer between shutdowns (or maybe even doesn't shut down at all) then you know it's heat.
If it still happens, then try disconnecting the laptop completely from the internet and just using it for non internet things. If it stays alive, then it might have something to do with your interned adapter hardware. Try using a pcmia adapter for the cable or wifi connection. It might cure the problem (it did mine!).