crashing

[billowhat]

this is a plea for help. My macbook (os x 10.4.7) is just shutting down unexpectedly. I have run clamxav & cleaning agent x.sit and no viruses have been found (as was expected). but i can't think of any applications or software that would be causing this. Sometimes it will immediately re-crash before the system even has time to boot up right after i press the manual start-up button after a crash. i don't think over-heating is the cause either because the underneath does not feel excessively warm after a crash. what's frustrating me so much is the lack of a pattern here. I have enabled the crash logs and they haven't helped at all. If anybody has any suggestions/solutions, please help and thanks