I had the exact same problem. I followed the advice of others also experiencing that exact same problem and created a virtual Wi-Fi network with Wi-Fi Multimedia (WMM) DISABLED. I created a virtual Wi-Fi network because disabling WMM really isn’t what you ordinarily want to do.
When I then bought some LIFX smart lightbulbs a little later, they ALSO kept dropping off, UNTIL I moved them onto the WMM-disabled Wi-Fi network.
I wonder if OpenSprinkler and the LIFX bulbs use the same chip.
@Ray, if you are interested let me know and I’ll share with you my exact router details and the firmware it’s running. If I enable WMM then boom problem reappears, if I disable it then voila the problem is gone.