Moses was deeply concerned about his people, Israel.
When he was about 40 years old Moses thought that God may utilize him – also taking advantage of his favourably position inside the Egyptian court – as a medium of salvation for the Israelites (Acts 7:23, 25). 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’ [δυνατος, 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 in the Egyptian court, directed – mainly - to benefit the people of Israel (not to mention the other princely activities Moses performed according Josephus).
Like we know, after about 40 years (Acts 7:30) Moses encountered Yahweh God (burning thornbush story) that commissioned him to serve as deliverer of his people. In this occasion Moses try to avoid this huge responsability pleading God a speaking problem (literally, ‘heavy [כבד] of mouth and heavy [כבד] of tongue’), as Exodus 4:10 affirms.
According LXX, Moses defined himself (in Exo 6:12) αλογος, ‘without a word’ (Emanuel Tov [2001, p. 126] translates αλογος as ‘lacking verbal fluency’), to corresponding the Hebrew expression שׂפתים ערל, ‘uncircumcised of lips’ (MT). Regarding these Hebrew idiomatic expression Barnes wrote (bold is mine): “An uncircumcised ear is one that does not hear clearly; an uncircumcised heart one slow to receive and understand warnings; uncircumcised lips, such as cannot speak fluently.”
Interestingly, Moses speaks about his tongue problem also in Exo 6:30, but in this instance, to correspond to the same Hebrew expression שׂפתים ערל, ‘uncircumcised of lips’ (MT), the Greek LXX hasn’t the term αλογος, but has the term ισχνοφωνος, that means ‘which has difficulty to speak’. This compound word is formed by two different terms, ισχνο that means ‘weak, reduced, unadorned', and φωνεω, ‘to emit sounds’'.
Some people see a discrepance between these two passages, but, is it necessary argue so?
We have to think about this, 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.
The spirit of Yahweh God, however, allowed Moses – from the age of 80 to 120 years old – to regain the flowingness of language he had 40 years before having received the deliverance divine commission.
The point to point for 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.’
I hope this answer your question.
