Please make sure the GND on VTx and Split is correctly connected:
And please also try below ways：
1. power your Split via USB with the TV-out cable and get live feed via this cable：
2. solder one or several low ESR capacitors (making sure it reaches 1500-2000uF）to the power source of the Split. For example, if you power the Split by PDB 5V output, you may add caps as below:
3. Change to another VTx. Some VTx is noisy in our test.
4. Add an insulating mat between the mount of the Split and the frame
Pls check the link for reference: https://www.rcgroups.com/forums/showpost.php?p=38034969&postcount=543.You may also insulate the antenna which is conductive form the frame.