Yes the resistor is in series. Very interesting, thank you.Depending on how long your line is, you could be suffering from oscillation between the source and line. Every line has a capacitance and if you switch a voltage vary quickly into that line, a current spike can be created. What it seems like is happening is that spike is bouncing back and forth between the line and your source; by adding the resistor (in series right?), you are slowing the rate at which the capacitance is charged, reducing the oscillation.