You need to enable SyslogCollector debugging in Collector.properties. Then restart SyslogCollector. When the messages stop getting processed, check the SyslogCollector.log for errors. This may not be the location of the problem, but it's a good place to start.

Note: I am assuming that when this problem happens, the messages are still being delivered to the syslog log file on the server. If not, then perhaps you have a network problem.