What apps are being used at the time of the crash/es you see?
Have you tried checking the hard disk for errors vioa disk utility? These can cause strange issues with the system and are one thing to rule out as a cause.
How much ram does your mac have installed?
Do you have any third party add-ons? eg: application enhancer, unsaity haxies, these are a source of issues at times also.
Is there any other things that seem strange when the crashes occur lets know as more...