2x4 Automatic Transmission need to rebuild or possible to fix

What is your question?

This happen from 2013 when the milage 170,000, when I change the shift from D to R and back to D again, there is a delay like 5 second. I chek the oil, it was black and I did change the oil and put the transmisiion cooler to keep the heat low because I do not have the problem at all in the morning , and it is help me to drive the car another 20.000 until today Jan 2015. But since last week the delay getting longer and some time need to wait until the oil cool.I need advice how to solve this problem, thanks.