Hello Richard,
Remove the small access cover under the tunnel carpet in the car. Observe that the three position solenoid on the transmission is working by watching the linkage on the tranny operate. It rests to the rear at idle centers when the accelerator is moved and goes full forward when the accelerator is pushed to the floor and rests on the kick down switch (engine off, ignition on). You may want to double check this function on a test drive with a second person.
If the linkage has gotten stiff, you may have free it up. Also any changes in engine vacuum from ignition timing etc. will also change the vacuum at the transmission and effect shifts. If you discover no issues with transmission or engine linkage and the ignition timing is ok, I would do a filter and fluid change. While the transmission pan is off, check for any debris in the pan and also check to make sure that the 10mm bolts holding the vavle body in place are all tightened to specs. Use a new cork gasket on the pan. Make sure to use the correct fluid, and drain the torque converter also. To refill the transmission fluid, start off adding 4 qts of fluid, start the engine and move the shifter through the gears (allows fluid to fill the converter). Begin adding more fluid. Carefully monitor the level as you add it is easy to overfill. Make sure you are on level ground with the engine at idle. When you reach the lower mark you may want to take a short drive (block or two) and then re-check and add. The fluid level will rise as the engine and trans warm up. At this point the level should be at the top mark. If you over-fill, it is easy to fill bleed some off by loosening the transmission oil line a the radiator.
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