Here is my suggestion: when it loses response, check the LCD screen and see if there is an arrow icon displayed at the right end on the second line. If you see that arrow, it means the controller is trying to ping the router but cannot reach the router.
If there is no arrow, try to click button B1 or B2. If the LCD does not respond to button clicks, it’s likely the controller is busy waiting for NTP sync to return, or waiting for weather / time zone query to return. These are likely due to issues with the Internet connection (or in case you have installed a firewall, perhaps the firewall is blocking incoming packets to OpenSprinkler for some reason).
If the LCD does respond to button clicks, then it’s really unclear to me why it has intermittent connection issues. We may have to ask you to send it back to us for further checking.