Some times there could be interference on the line, or a bad neutral connection, bad ground conn. http://www.mygreenlee.com/products/det.cfm?id=3195&upc=00521 this is the tracer I have, and while I have run into the same problems as you have, for the most part it has paid for itself over many times. Sorry I don't have much more info to give, but it is always a good tool to have.