I would start by looking at the input to the 1117 on a scope to see if the buck regulator is somehow misbehaving. Then check your connections, such as the polarity of the filter caps on the input and the output. What about the length of the wires from the caps to the 1117? Are they extraordinarily long? As for using an inductor, don't! The 1117 is not designed to be used with a series inductor. That will just complicate the situation, and you really want to keep it simple.