1.2g video transmission interefering with 2.4g spektrum

As my FPV sometimes get no response or clumpsy response in air, I determine to range check all my receivers with video on.
Gears : DX7, AR 6100, AR 6100E , AR 7000
Ground test result:

1) 1.2g video transmission on and with spektrum DX7 binding switch pressed: (Factory recommended range ~30steps ~90feet)
AR 6100, 6100e - less than 15 steps (40feet).
Ar7000 - 30 steps (90 feet), no more or less

Binding switch not pressed : All have range >90feet.

2) The test was repeated with the video transmission power off, and binding switch pressed:
All have range >90 feet

Switching on sequence - 1) video transmission, receiver, 2) DX7, 3) RC recevier
Different video transmission channel was tested. Some channel has worse gliches (those close to 1.2g, e.g. 1.17, 1.20, 1.21) and shorter range.

Weird enough, in ground test, the out of range may result in knock off (no control at all) for 4-7 seconds, till the DX7 is move closer in range and wait for 4-7 seconds. It seems it is not simply out of range. I guess the receiver is masked by the strong signal from the 1.2 video transmitter and drop it is channel and run to seek for new channel again.

This concord with my experience of occasional clumpsy control !