The majority of Source Engine crashes are due to plugin errors and exceeding engine limits on maps. When you're faced with a constant crash on your server, there are a few steps you should take to pinpoint the culprit.
First ask yourself a few questions: Does it always happen on the same map? Were players in the server when it crashed? Does it crash on startup or on map change? Once you have the answers to these questions you can then start troubleshooting. First off, try running the server with no plugins. This means don't load metamod, sourcemod, Eventscripts, etc. Let the server run like this for a few days. If there are no crashes then you've at least pinpointed that its not a corrupt installation. If it still crashes, than either you have a misconfigured CVAR or your installation is corrupt (delete all steam files and re-run the update process). You will know if there is a crash by looking in the root directory of the server for MDMP files, MDMP files are created whenever you server crashes and are created at the time of the crash. You can't see whats in the MDMP file though, only the developer of the mod or game can. MDMP files are just usefull so you know the date and time of the crash.
To troubleshoot plugin problems, first try running the server with the default plugins that came with your administration tool. Than every day or few days after that add 3 to 6 of the custom plugins you are using. If the server crashes again, than its likely one of the last few plugins you added so remove a few of them and let the server run for a few days again. This method can take a few days to a few weeks, but its the best way to find the plugin that is causing the crashes.