Most likely your processor is going bad. If you are experiencing frequent crashes, that usually is the culprit. Also if you get any errors at startup such as "...has detected a problem with the built-in cache memory" , then that is another sign it is the processor, but you don't have to have both symptoms...
Processor replacements are available from the link below:
http://www.powerbookmedic.com/xcart1/cu ... hp?cat=340