My Spark Core was working fine for weeks sitting next to me in my office (20 feet from the WiFi access point). Then one day it started dropping the WiFi connection and could not recover it. After a lot of searching I decided to move it closer to my WiFi access point (8 feet) and it worked reliably again. Weeks later I moved it back to my office and it worked fine for a few days; then it did not.
My assessment: In my office the WiFi signal is strong enough most times, but occasionally someone/thing near by our house starts transmitting (perhaps the police station?) in a way that prevents my Spark Core from connecting when it is 20 feet from the WiFi access point.