F5 unexpectly monitor pool member up/down many times / day

Well to state the obvious, the server(s) did not respond with 16 seconds to the monitor check. My guess is that the server(s) are busy and can't respond. So you can increase timeout, or you can increase the interval and the timeout.

Do you need 30/91? Well, you need to decide how long you want to send new sessions to a server that could be down. Do you want to do this for 91 seconds?

You could try using 10/31 and see if that helps, that would just about double the time you have now.