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The most common cause of errors like that is a plugin. Are you running plugins, and have you tried temporarily disabling them to see if the error stops occurring?
If it's not a plugin, it could be that someone is using an exploit on you. There are several out there that Valve hasn't patched yet.
If temporarily disabling plugins does address it, the most likely culprits are the old SM version (I'd recommend upgrading this to 1.2.4) and Mani mod (you should remove that, if possible).
Well the crashes seem to have stopped. Is there any way for the server to automatically reboot if crashes do occur in future? Rather than me doing it remotely everytime.
Here at NFO, we have a script that checks each server for responsiveness every 5 minutes and auto-restarts them remotely, which also would take care of it for you but would be much more complicated to set up.
You should also make sure that you have the various error reporting dialogs turned off in Windows, through the Control Panel->System->Advanced->Error Reporting box.
We run mostly Windows servers and I've definitely seen errors pop up even when that option is used (such as in response to a couple of CS:S exploits that are currently making the rounds). But, it's very likely that it does prevent many of the more common types of errors, so it's good to have on there.