I'm not an expert, but is your phantom a 5.8ghz edition phantom???
Cause if it is..... I can tell you that the FPV fat-shark system your using is also a 5.8ghz system and it will take over the signal of the phantom and send it to wherever it wants to go because your FPV transmitter is pry telling it just random signals and the phantom gets confused and takes off until it runs out of battery life. This happens because your using the same 5.8 ghz frequency and typically FPV systems have a stronger signal then the controller and phantom receiver.
Cause if it is..... I can tell you that the FPV fat-shark system your using is also a 5.8ghz system and it will take over the signal of the phantom and send it to wherever it wants to go because your FPV transmitter is pry telling it just random signals and the phantom gets confused and takes off until it runs out of battery life. This happens because your using the same 5.8 ghz frequency and typically FPV systems have a stronger signal then the controller and phantom receiver.