The real reason behind Goku's power
Some people think Goku had more power when he was angry before he turned good that might be true.
But the right reason is that Goku's power comes from his bravery if goku's scared he looses his power.
As pan once said to goku's great great grandson(dragon ball gt movie 15), goku your power comes from your bravery like your ancestor goku.
he was a hell of a fighter but he was never afrad that's how he won his fights.
The end