Have not tried using SD card on this model yet, but would be good to check the speed class of the SD card, especially if the 1920x1080 @ 15fps is used, might be hitting limit on burst/sustained writing speed of your SD card. The reason for mentioning this is that I initially had the DCS-8526LH on Wifi (AC1200 Access Point) with 1GE link into the rest of the home network to write to the Synology NAS (1 additional switch with all 1GE ports), this could not support sustained recording either, until I used physical Ethernet cable to connect the camera to a 1GE port on that same AC1200 Access Point, trouble free recording after that.
