I am not sure which version of Call Manager you are using, but in 4.0(1), there was a issue of RTMT sending low heartbeat.
To confirm whether this is an actual issue, you can use Perfmon to log the heartbeat counter for the service every second. Once there is an alert check the log file to see if the heartbet counter stopped incrementing at the time of the alert.
The hearbeat rates are as follows:
- CallManager 1 every 2 seconds
- TFTP 1 every 1 second
- TCD 1 every 1 second