1. Wireless network.
2. 2.4 GHz corless phone.
3. Microwave oven.
Interference from these is less likely with the Video Sender PRO, which is supposed to be 900 MHz.
They did that on purpose, to move away from the more crowded 2.4 GHz frequencies.
The suggestion to change the channel on the sender and receiver is probably the best place to start.
that, and adjusting / re-orienting the antennas so they are as lined up as possible.
Look for other sources of 900 MHz interference:
older cordless phones
wireless intercoms
baby monitors
Try to track when the problem happens, and see if it is a specific time of day. It could be interference coming from something in a neighbor's house, too.