This is what's known as 'server flapping.' This occurs when healthcheck settings are too aggressive and incorrectly marks real servers as down when they are not failing. This can be eliminated by changing the healthcheck settings in Rules and Checking > Check Parameters > Service Check Parameters.
Check Interval (sec) The time in seconds between healthchecks.
Connect (sec) The time in seconds before LoadMaster abandons a healthcheck.
Retry Count The number of healthchecks that must fail before LoadMaster marks a server as down.
LoadMaster will check every Check Interval seconds to see if the server is up. It will wait Connect Timeout seconds before giving up. If it gives up, it will retry Retry Count times before officially marking the real server as down.
To reduce false negatives when healthchecking, you can change these values to be less aggressive. Please note that making them too relaxed may result in users being sent to servers which are not responsive since it will take longer to detect a failed real server.