Car has been throwing a yellow warning light for quit some time which has been ignored for some time without knowing what it is. Been reset several times. Just lately the tranny has been "stuter" shifting down from 4th to 3rd as the speed slows just a little but not much. This must have been the problem but not very noticeable. Lately it has gotten worse and a few times tranny has stuck in 3rd. Have had to pull over, turn off the car, and then restart it and the problem goes away. Anybody experienced this problem? I suspect that it is a shift solinoid in the transmission going out?

BTW, I run an automotive shop (Jeep & 4x4s) and change the tranny fluid about every 20K miles and use Royal Purple ATF.

 

By searching Royal Purple is fine with your tranny. Your Vehicle is ATF M-V, so Royal Purple does cross to that: Max ATF® . Good job on changing fluid frequently.
Mileage?
I would seriously first consider put a ground wire from the battery to the tranny, especially since it had to be re-set.
Other grounds i recommend: another one to the body, 1 to engine block and 1 to throttle body. Changing a tranny solinoid is serious $$$$$, it would be worth a try.

If a late model transmission (computer-controlled transmissions started becoming popular in the early '90s) is not shifting properly, it is often the result of a computer sending incorrect signals due to a faulty sensor, or the transmission is not reacting to the computer command because of a bad connection or defective solenoid pack.

The TPS is a three wire sensor that measures the throttle plate opening as well as its rate of change. This sensor is a variable resistor, also called a potentiometer, that is directly linked to the throttle plate shaft. The TPS outputs a voltage directly proportional to the throttle opening. As the accelerator is depressed the throttle plate opens and the TPS voltage increases. This sensor is also one of the main inputs to the transmission computer (TCM). The TCM uses the TPS input signal to control the transmission shift points and the torque converter (TCC) solenoid lock-up.

FOUND THIS TO BE INTERESTING: When you talk about erratic trans shifts, exactly what is it doing? I may be having a similar problem and hadn't considered a voltage problem yet. Had the trans "flashed" in hopes it would fix it but it didn't.
Tranny was going in and out of lockup in 3rd and 4th. Problem is caused by a multitude of problems:
1) Bad grounds off of battery to body, frame, engine and ECU to body.
2) ECU voltage regulator for alternator.
3) Diodes in alternator.
4) worn alternator brushes skipping on commutator in alternator.
5) bad wiring connections on back of alternator.
6) Cleaning the grounding points to include sanding the paint off of the body, frame and engine were grounds connect helps alot. Also the grounding bolts/screws get corroded and need replaced.
If its alternator related, the big wire coming off the altnator going to the ECU runs through the wire loom with all the wires that are coming or going to sensors and back to the ECU causing the sensor wires to pick up stray voltage spikes including the wires coming off of the APPS and will cause tranny to go in and out of lockup.
ATS has a page were it shows rewiring the APPS wire back to the ECU, allowing the wire to get out and away from the alternator wire in the wire loom that may be causing voltage spikes.
ATS and other compainies like BD diesel sell a noise/sponge isolator that I believe is nothing more than a Capicator that can hook into the big wire off of the alternator or the APPS wire going into ECU. This isolator basically will absorb any high voltage spikes and keep the ECU and tranny happy. Not a bad idea but if the alternator is the problem and its caused by worn alternator parts why not fix it. If the alternator regulator in the ECU is bad then the alternator would be charging at very high voltage not sure how high but I believe higher than 16 volts.
If a diode was out than it can also cause the alternator to charge high but also if the truck sits for awhile it will drain the battery. Trying that now, going to let the truck sit for a couple of days and then check battery voltage.
If the brushes are worn it will cause them to skip on the commutator and cause voltage spikes and the brushes need replaced.
For me; if any part of the alternator is bad I'm just going to replace it with high amp alternator because of the big winch I'm going to install, besides I believe the alternator is the original and I have 142K on the engine.
I also noticed while cleaning all the grounds that Dodge used small ground wires going to the body so I'm adding bigger grounds from the engine to frame and to the body.

Mazda Grounds are minimal, extra ones help!

 

About 175,000 miles on it. Thanks for the response, will give it a try. Hope its one of those things. I'll let you know.