You need to use a lower frequency that will go through foliage and "around" obstacles.
Is the ground level? Can you maintain line of sight?
Separate the video stream from the control. Use model car radio control devices on 53 and 75Mhz for control and 2.4Ghz video for vision. This way, you can lose the vision part of the link, and not loose control of the robot. If you drive around an obstacle (i.e. vehicle) and lose video, you can then move the robot to a different location and regain video contact.