The right answer would be something like, "oh, I've come across this specific problem a lot. I found that all you had to do was add x, y, z components at p location in your diagram and that will prevent the problem."
If your not sure if the voltage is dropping or not, use a volt meter to confirm that. However, you did say that you put the video monitor on a different power supply and it did nothing, SO maybe voltage drop is not the problem. You are most likely having a shielding issue. Do like G_Mike says, and rewire it correctly. If you want another way to test the RF from the motor, is to simply use an AM radio. Set the radio to a clear station, and try to turn the motor on and off. If you get interference when the motor is on, then you need to shield the motor. My guess is the motors metal casing is not shielded. What you can try is run the motor with the radio on, then use a wire to touch the metal casing to ground. If the interference goes away then you know you have a shielding issue.
It's a 90W computer power supply and the motor is a power door lock actuator.
yup, but I still need a definitive test that tells me that the problem is too much voltage fluctuation. It happens to fast too be detected on my digital meter...
If I introduce a second power supply that shares the same ground, and use that to actuvate the power door lock motor, the video loss problem is gone.