Why is my brothers Buick shifting hard and acting like loss of power when I press gas pedal

What is your question?

When I drive my brothers Buick after starting engine I put it in reverse and sometimes it acts like there's a power delay because it will all the sudden get all the power it didn't get at first and then I have to put it in 1st to get going and then shift on the fly to 3rd for cruising speed and do that at every red light or near stop. If I put it in D (overdrive) it doesn't idle or cruise like it should. This happens about 70%-75% of the time when it gets drove. It also has a slow oil leak but I'm sure that's not the problem.