평소에는 맨날 어버버- 하면서 가끔 보면 말을 참 잘한단 말이지? translated to english???
My rough translation of this is something like:

Every day as expected/usual - sometimes I see you you speaking very well.

My understanding is that the person finds that other person says something impressive/surprising once a day.??

Question

평소에는 맨날 어버버- 하면서 가끔 보면 말을 참 잘한단 말이지? translated to english???
My rough translation of this is something like:

Every day as expected/usual -  sometimes I see you you speaking very well.

My understanding is that the person  finds that other person says something impressive/surprising once a day.??

Answer

어버버 is an an onomatopoeic word of stuttering.
어버버하다 is a colloquial expression. It means "(말을) 더듬거리다"(stammer, stutter)

평소에는 맨날 어버버하면서 가끔 보면 말을 참 잘한단 말이지.
Most days you usually are stuttering , but sometimes I can see you speaking very well.