Moses was deeply concerned about his people, Israel.
Maybe, he thought that God may utilize him – also taking advantage of his favourable position inside the Egyptian court – as a medium of salvation for the Israelites (Acts 7:25). This situation happened when (Acts 4:23) he was about 40 years old (or less). In that epoch Moses was “a man with power both in his speech and in his actions” (Acts 7:22, NJB).
According the chronological sequence of the happenings described by Stephen, this Moses’ ‘power’ [‘dunatos’, Greek] was not in reference with the subsequent encounters with the Pharaoh – besides, Stephen did not mention that encounters between Moses and the king of Egypt – but refers to the Moses’ strong management of his superior position when he was in the Egyptian court, directing, mainly, to benefit the people of Israel (not to mention the other princely activities Moses performed according the recording of Josephus).
Like we know, after about 40 years (Acts 7:30) Moses encountered IEUE God (burning thornbush story) that commissioned him as deliverer of His people. In this occasion Moses try to avoid this huge responsibility pleading God a speaking problem (literally, ‘heavy of mouth and heavy of tongue’), as Exodus 4:10 states.
Some people see a discrepancy between these two passages, but, is it necessary argue so?
Ask ourselves, What kind of life Moses had in the intervening 40 years?
From the few clues the Bible offer us we may conclude that – after his departure from Egypt – he drastically changed his life-style: from a princely status (‘son of Pharaoh’s daughter’), as a powerful man belonging to royal entourage, to an humble sheep-leading life, as shepherd of a flock not even belonging to him. Further, the years went on also for him. Perhaps, in that intervening 40 years, he lose gradually the breeziness he had when he was in Egypt court, losing also the habit to speak in public as he did before.
I think there is no contradiction between these two Bible passages.
The 'point to point' this story teaches us is to trust fully God, without a doubt.
Like a pastor said many years ago, ‘When he (Moses) thought to be ready to deliver Israel, God thought he was not. When he (God) thought Moses was ready to deliver Israel, Moses thought he was not.’
This is in perfect harmony with the Paul's discussion on his own weakness. What God said to him, about it?
"My grace is sufficient for thee: for my strength is made perfect in weakness." (2 Corinthians 12:9, Webster)
As always, IEUE God was right.