As no other servers are reporting this, it could be a problem with one of your scripts. I suggest:
* Updating your server to the latest stable: http://linux.multitheftauto.com/
* When the problem occurs, check the server log for script errors, and also the client file MTA\clientscript.log
* Stop each script in turn to see if it fixes the problem
* Check your race script for any modifications that could be causing problems