strange phenomenon in spectrogram of Windowed Fourier Transform [edited]
This happens because your window is too short. I don't have access to a plotting tool right now, but imagine for a second a slowly varying sinusoid that you chop up into pieces, and these pieces are shorter than the period of your sinusoid. If you take the Fourier transform of each of these pieces, some of your chunks will capture more energy of this sinusoid than others because they will contain crest or through of the function, while others will contain regions close to 0.

If you increase your window size, you'll be able to catch several cycles of your slow sinusoids inside each window, and the periodicity will go away. The tradeoff is that your windows will capture larger lengths of time, so boundaries between events will be more blurred.

If this still doesn't make sense, I'll try to include some plots later.