ESP8266 - UDP send interruptions
I think you need a delay(1); after your call to write().

There is/was a bug where the underlying buffer files up and can't get time to clear.  It looks like it will require a change to the ESP8266 firmware from the manufacturers before it will get fixed properly.

See what you think:

https://github.com/esp8266/Arduino/issues/1696

https://github.com/esp8266/Arduino/issues/2285