We have exactly the same issue. The F5 stops doing the health checks for no apparent reason (marks the node as down) and then resumes the health checks after about 3 minutes (174 sec). When the F5 resumes the health checks the nodes is marked as up immediately.
I've got the tcpdump traces (taken on the F5) that confirm the F5 isn't doing any health check during the outage). I've also got the bigd log and there are no failed health checks in there.
Did you log the issue with F5 and got a patch or workaround?
Kind regards,
Jan