It sounds like you have a 440-T4, which does not have solenoids. Code 39 definition is "torque converter clutch engagement problems", AKA, no lockup. But if you are not going into 4th, the computer will not see the correct RPM @ cruising speed and blame it on the TCC.
Try this, disconnect the modulator valve and the downshift cable and start the engine. Now pull the downshift cable all the way, a few times, very quickly and connect everything back up and drive the car. If it now shifts fine, you probably have a sticky TV valve, which could be caused by a failing transmission/metal contamination.
Remove the pan and see what is sitting there.
1957 Nash Metropolitan
1960 Bonneville Conv 3x2 1964 Bonneville Vista
1965 Bonneville Conv
1965 2+2 455 3x2 and 1966 421 2+2 3x2 (donor car)
1967 GP Conv