What is the most likely cause of erratic readings on an SWR meter?

If an SWR meter's needle jumps around or reads erratically, it means the impedance is fluctuating rapidly while you are transmitting. This is the classic symptom of an intermittent electrical connection, such as a loose connector, a broken wire swaying in the wind, or a poorly soldered joint somewhere in the antenna system. When the connection makes and breaks contact, the reflection of RF power changes instantly, causing the jumping needle. Transmitting into the wrong antenna would cause a steady, albeit high, SWR. Interference from power lines usually affects reception noise rather than your transmitted SWR.