While recording at 120 fps H.264, D-Cinelike; standard,  I experienced on two different brand of micro SD cards what I would call video jumping back 1 frame and then continuing.
Has anyone experienced this with the X5S camera or if possibly the micro SD card. Is there a preferred card to use while shooting at 120fps?

Si está intentando grabar un video 4K (XAVC S 4K - 3840 x 2160) a 24 cuadros por segundo, proporcionará una tarjeta de memoria que admita los 100 Mbps requeridos.
Cuando intente grabar en HD (XAVC S HD - 1920 x 1080) a 120 cuadros por segundo, en una tarjeta de memoria que admita los 100 Mbps requeridos.
Tuve un problema similar cuando instalé algunas cámaras CCTV en mi empresa y resolviendo el problema Micro SD clase 1.

I use Lexar MicroSDXC 128GB 1000x microSD 150Mb/s Class 10 UHS-II U3 for high bitrate.
Are you shure that issue is in SD-card? How you detect the drops?