I had Transmission changed in 05 Lexus es330  It shifted fine for 2 days then while driving it the car just suddenly down shifted on its own. I was told the computer has to relearn the transmission.  Now, my sons driving it and tells me the car shifts fine but at 20 miles per hour it has a slight jerk then after this he's had it up to 80 and it wants to run like a great car.

 Question is what can be done to correct the issue ? I watch many of your videos and sometimes you say the computer has to relearn the Transmission. It scared me bad when I just had the trans put in for over 1k then this happens.  Is there any additive to add that will help the shifting?  I did find out the Transmission I bought sat for about 6 years and there was no fluid in it when I bought it.  I need help badly. I bought the Lexus because you talk them up as great cars. Over all the interior is beautiful and out side looks like it may have a slight side swipe but nothing major . I had all the Transmission sensors changed thinking that was the problem. The place that put it in said they took my new ones out of the other Transmission and put the in the used one I bought.   

also,  you speak on putting a transmission additive in many Lexus sedans.

265k miles engines no burning oil, runs great except that one issue.

What's the recommendations  on any help thanks