Even after three signs, Moses refuses. Targum Pseudo-Jonathan preserves the protest with a phrase more vivid than the Hebrew: Moses is of a staggering mouth and staggering speech — or, the Targum's alternate reading, lame. His tongue does not walk straight. His words lurch.
Moses piles the excuses: I am not a man of words, nor ever have been — before this conversation, during this conversation, and, the implication goes, after it. The Targum sharpens the complaint by adding before that Thou didst speak with Thy servant. Moses is essentially saying: the burning bush did not cure me.
Why the Liberator Stammers
The sages of the Targumic tradition read this moment with compassion, not frustration. The liberator of Israel is a stammerer. The man who will one day deliver the longest speech in the Torah — the book of Deuteronomy — begins his ministry insisting that he cannot put two sentences together.
Is this false humility? The Targum does not think so. Staggering and lame suggest a genuine speech disability, which later midrash (Shemot Rabbah 1:26) will trace to the incident with Pharaoh's crown and the hot coal.
The takeaway: the Jewish imagination refuses a flawless prophet. The man chosen to speak for God is a man who struggles to speak at all. The Memra — God's Word — will supply what the stammer cannot. Moses will deliver the Torah not because his mouth has been cured, but because he finally agrees to open it anyway.