Troubleshooting Pool Members failing HTTP Monitor

Hello, 

You should perform packet capture on the F5 unit and on the server, and compare the times between F5 unit sends probe and server receives it, and when server sends response and F5 unit receives it. (also check, is response delayed on server already?)  

If there's a big latency in monitor response, and responses are often received after timeout time, consider tuning your probe and timeout timers so that server will have enough time to reply. 
You can maybe issue probes manually with specific settings so that they will be easier to extract from pcap file.