There are 4 possible reasons for this issue.
1. Split's input current could be not enough with the decrease of battery voltage on the flight. If PDB/BEC/FC/VTx can't supply enough current to Split, this issue will happen. Split's average working current is about 650mA. So please make sure input current is enough for it to work normally. It is better to power your Split via BEC(5V >=1A) / PDB 5V >=1A / FC 5V >=1A.
2. Ribbon cable doesn't connect well. Please try another ribbon cable(Click How to replace lens cable of Split for reference, please) or try to attach thin tape to the below part on both ends of the cable:
Please fix the connection with electrical tape of the below two parts as well after the cable is well-connected:
It also might be caused by shaking of your Quad:
1) Please mount on new props，or change to other brands, refer to goo.gl/nEY316, #807 and #808;
2) Adjust PID, refer to goo.gl/o121wP
3) Calibrate the Throttle Range of your ESCs, refer to goo.gl/7z8rCV
4) Soft Mount motors, refer to goo.gl/JYGLEQ
5) Upgrade the firmware of your flight controller
3. The SD card's slot connector is not in good condition. Please refer to:
Split's video is choppy or with a low fps during the flight, also no reaction while pressing the buttons
Split says "No card" while flying and FPV image is choppy
4. The speed of the SD card cannot meet the requirement.
Which type of microSD card is fit for RunCam Split?