Flushed the trany multiple times with no success. Auto 2006 MCS r53.
Fine when cool then after driving 5 miles hard down shift horrible automatic 2nd to 3rd gear shift. Screeches the tires. When I drive in Manuel shifts fine,
any ideas besides driving it in a lake and collect gap ins?

 

You have the typical symptoms of a valve body failure. There is a large thread on here regarding swapping fluid and valve bodies. Check it out as it will be very helpful if you are a DIY type person.