Back every thing up, Boot into Safe Mode and let it run overnight to see if it crashes that will help isolate hardware vs software issues. Run Malwarebytes and AVG in safe mode. You might also have a boot sector virus, use the fix MBR command.

If it's hardware...could be anything. Note really worth the effort when a new motherboard CPU combo can be had on newegg for $114 <img src="/forums/images/graemlins/cheers.gif" alt="" />

98 Montero with cold weather package
96 Toyota Land Cruiser, fully locked Mall Machine :-)