Hi Deannda,
Each pixel on the sensor stores electrical charges in response to light, and the camera reads the amount of charge present as the pixel value. Charge can leak in though, and some pixels leak more than others.
All cameras have this to some degree. The trick is minimizing it. In order to do this,k make sure you shoot at the lowest ISO possible. If the ISO is set to "auto", the camera may be increasing the ISO without you knowing it. Try shooting in a manual mode, at lowest ISO and see if this helps.
ALso, many cameras have a noise reduction function that compensates for them somewhat, by doing an automatic "dark frame" subtraction. If you have this function, make sure it is enabled for slow shutter speeds.
I hope the above helps, PM or e-mail me if I can be of further help.
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