Sounds like you have one of the older, HDMI 1.0 Atomos cables. The same happened to me — I didn't realise that there were two versions out there. The only Atomos cables rated for 50/60p are the current, $90 ones.
Also, regarding the GH5S stuttering issue in this thread, I am experiencing it even when recording 1080p at 29.97 to the Ninja Inferno. The same behaviour as described in previous posts — the occasional duplicated frame in place of the correct one — e.g. a sequence of four frames 1-2-3-4 is recorded as 1-2-2-4.