If your monitor has been replaced to eliminate any possibility of a defect, but you still encounter interference problems, then your monitor would be considered non-compatible with your surroundings.
There are several items within the household that will work on the same frequency as your monitor. This could cause undesirable interference and in these cases, you may have to eliminate these items in order to allow your monitor to work. Outside of your home - items your neighbor may have - that also work on a radio wave frequency can also interfere with the performance of your unit causing undesirable performance.
Some of these items can include (but are not limited to): Microwaves, clock radios/stereos, garage door openers, wi-fi/modems, cordless phones & cell phones, CB & 2-way radios, police scanners, satellite dishes, wireless security systems, wireless computer accessories (mouse, keyboard, speakers), invisible pet fences, and additional monitor systems.