You're sampling rate is too low, you're sampling at about 2.5 times faster than the input signal, if you draw a square signal on paper and then place dots on it where sampling is done, you'll grasp what's happening.
Don't forget to offset your input so it's always positive, arduinos don't like negative voltages.
I would do the serial print only once per second or you'll fill the buffer.
And increase the sampling rate to 2KHz and make sure the sampling interval is always the same (by calling micros() or using a timer)
Ideally you should use a low pass filter to remove frequencies above 200Hz but this can wait.