my A/T seems to function only with 2nd and 3rd gear, would not engage in overdrive. But in the morning or after the vehicle is rested the AT will function perfectly but after 15 to 20 minutes of driving, the problem will occur. ATF is ok, no burnt smell and replaced with dextron lll, filter is ok and the valve body is overhauled. Computer box was replaced.
Could it be the computer box? Is it programmable or simply don't match with the A/T?
Thank you....

AV