Interference between 27MHz RX and 5.8GHz TX
Interference might be caused due to non-rf components too.. e.g. power lines. Or, your Video-Tx might be eating a lot of power from battery causing voltages to drop somewhat (less likely)

Do you observe drop in range even when video tx is on, but not mounted on car?

Try separating Video + Video-Tx system from the rest by powering it from a different battery.

Also, keep video-Tx antenna as far away from RC-Rx antenna as physically possible.