When I first started using the cameras, I was having similar problems. Then I finally realized that my receiver sitting on top of the computer was probably not a good idea. I have since moved it up high on a shelf, with no electrical equipment in direct line of any of my cameras. This solved the majority of my signal problems.
I have 9 cameras, with a 180 degree spread between them, and one actually on the second floor (reciever and others are all on 1st floor). One of my cameras has too many walls and other obstructions between it and the receiver so I extended the signal strength by using another receiver/sender set. The camera and receiver are set to a different channel than the primary. The receiver plugs into the sender which is on the same channel as the primary. I control the sender by turning it on/off with an appliance module as part of the same code as the camera. Three of the cameras are the pan/tilt type, which often don't point the antenna directly at the reciever when they are moved. The furthest is about 35 feet away and transmits through 2 walls and a set of stairs.
The only time I get a slight problem, oddly enough, is from by backyard camera. It only happens when I'm sitting in my office (location of the computer and receiver), and I move my chair into a certain position. It's odd because the chair isn't in direct line between the two, however, since I don't have the reciever antenna directly pointing at the camera, I assume there is some bouncing going on that accounts for the reception as well as the minor interference that my chair causes.
I know this doesn't give you specific solutions to your issue, but hopefully it encourages you to keep trying and find the right solution for your setup. It is worth it once you figure it out.