This usually has to do with your particular network setup. Do you have just a single router or do you use switches, multiple routers, or have advanced settings like VLANs etc. The OpenSprinkler uses an embedded microcontroller (ESP8266) and it tends to be overwhelmed if you have a lot of network traffic.
We generally recommend users with wired Ethernet issues to file a support ticket at:
support.opensprinkler.com
because we need to know your specific setup, your OpenSprinkler hardware version, firmware version etc. Users who post on the forum usually don’t report such information making it hard to diagnose the issue.