Please make sure the GND on VTx and Split is corrected connected:
If the problem exists still, please power your Split with a phone charger(5V 1-3A) via USB to see if there is interference existed in HD video.
Then directly connect your split to a monitor like this:
If the interference disappears, it indicates the interference comes from outside. It's not the issue of the Split.
Pls try measures below:
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 board as shown below:
3. Change to another VTx
4. Change to another PDB
5. Add an insulating mat between the mount of the Split and the frame
Please 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.