Macbook Pro 2007 Crashes
It very well may be the graphics card in your MacBook Pro. Apparently during the manufacturing process (of this generation) too much thermal paste was applied to the heatsink on the GPU, thus cooking it. If this is the case for you, Apple might replace it or reimburse you for repair, if they are still doing this. 

A few good links:
http://support.apple.com/kb/HT4132?viewlocale=en_US&amp;locale=en_US
https://selfsolve.apple.com/agreementWarrantyDynamic.do