Every so often ESP8266 hangs... WifiClient seems to be at fault
It is a bug and it was fixed in Arduino esp8266 core package 2.4.2 published 2 days ago.

see first line in ESP8266WiFi section of the changes list


  2.4.2 ESP8266WiFi:
  Fix memory leak when reusing WiFiClient object (#4497, #4516, #4549)