Hi Gea-Suan,
I have had this problem ongoing on and off. I haven't been able to resolve every one one but about 95% of it was related to how the webserver responds to health probes. Most cases I deal with it was adjusting the probing from say 5 seconds to 10 seconds with a larger timeout and that appeared to either resolve it or usually cut down the false alarms. In other cases the application needed to be tuned. Ultimately your best bet is to work with F5 support so they can look into your configuration and determine the best course of action.
Bhattman