Slow take off

My 96 cutlass supreme sat for a year while I traveled. Before I could fix it.
Lower ball joint broke. Car repo man cam by one night and took car out from between two other cars. I had the title so he had to return it to me. I just wasn't home when they did this. He pulled the cv joint out from going sideways. I fixed all that and got car moving again.

Put in a new radiator. Filled it up. Checked all other fluids and made sure they were where they needed to be.

Started driving car. Right away I noticed it was lacking power from take off.
The car didn't want to go when in D. It took for ever to move. Like it was bogging out. Worked just fine if started from a stop in 1. RPMS were not right tho. It never goes down like it should when I get to the speed I want to travel. Then one day it just started working the way D is suppose to. Took off from stop like a brand new car. RPMS were where they should be.

Just the other the day the problem came back after going down the highway for 50 mins. Chilled with family. When I went to leave is when the problem came back.

I checked all the fuilds in car and added what it needed before I left.

Car is a 3.4 DOHC V6

Sounds like the trans is starting in 3rd gear default, without onsite road test this is hard to diagnose. Check the main connections at the trans, look for corrosion or looseness, you may need a scan for pending codes also.