If anyone can run WireShark could you monitor the network traffic for a while? That may help find out the issue.
Because OpenSprinkler runs a microcontroller-based embedded server, it’s not as fast and robust as a Linux box. Especially if it’s performing other web tasks (like it periodically pings the router to see if it’s still connected), it can’t respond to incoming requests. That could explain why the ping sometimes works and sometimes doesn’t. Also, it periodically send requests to the cloud server, so that will temporarily block the incoming requests too. The firmware does check the connection and when it decides that it has lost connection it will re-start the Ethernet controller. In any case, all I am saying is that it’s a much weaker server than a Linux box.