did Moses really have a speech impediment?

An interesting query comes from Gerald M. Siegel, professor emeritus in the Department of Speech, Language, and Hearing Sciences at the University of Minnesota. He writes:

In the Torah [Exodus 4:10], Moses initially resists being God’s messenger because of his speech, saying: “Please, O Lord, I have never been a man of words…. I am heavy of mouth and heavy of tongue.” From this the rabbis concluded that Moses was a stutterer, which a story in the midrash attributes to his having burned his tongue on hot coals as an infant. Yet why decide that he stuttered rather than that he suffered from some other speech impediment? On what basis was this explanation given?

"This is an interesting position because there's no where in Torah stating Moses having any issues speaking to the Israelites or God"  

http://m.forward.com/articles/142439/did-moses-have-a-speech-impediment/