Check fluid level while in park and warm?
What is the age of the trans? Is it high mileage ?
If fluid is fine as you post, it Sounds like this problem developed as the cold temps arrived this year?
Could be old tired , hard seals inside the trans. Refered to as "morning sickness"
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.