I have had the same problems,Are your cameras CMOS or CCD, 1/4" or 1/3". I now use 1/3" CCD which are better quality. I also started off with ready made av/power cables from ebay which were ok but not brilliant, an improvement was made by using rg59 coax/power cable £30/100m on ebay .
The other problem I found was street lighting which affects the auto iris, so avoid pointing a camera towards a light.