I have an 04 3.0l with 109k miles on it and believe that my transmission or torque converter may be acting up. The fluid has never been fully changed but I have had to add about a quart before from adding a temp. gauge. Would I be better adding some additive (what brands would you recommend) and leaving it sealed, changing the fluid and filter, adding some cleaning and driving then changing the fluid and filter, or even taking to a shop and have them do one of those cleaning things and let them change both. Opinions?

For some reference I believe my torque converter may be locking up. My fluid looks good and the transmission seems to shift smoothly otherwise. The temps are normal and the truck has had an aux cooler it's whole life but does pull some heavy loads for it from time to time.

 

So engine starts to stall when coming to a stop?
i.e. torque converter not unlocking at lower speed

This is controlled by the computer via a solenoid inside the transmission.
Computer bases its lock/unlock on speed, RPM and throttle position.
The usually problem is that the electrical connections inside trans start to fail.
But worth a look at the outside connection, unplug it and make sure all contacts are clean and dry.

Lucas makes an automatic trans additive that helps expand seals in the trans.
Automatics run on pressure, valve body seals start to leak and pressure drops a bit, additive helps to keep the pressure up.

What temp is the fluid at when driving?
Just under 200degF is where you want it, 225degF max when towing

Not sure on the full fluid exchange, really up to you, I have always been happy with just regular filter and fluid service

 

No mainly shuddering at speed and possibly when going in and out of overdrive. I have been told that this could be caused by the torque converter unlocking and locking and or some solenoid sticking. I've never been a big fan of doing anything that messes with seals... and my average on my gauge is 120-160ish (thats on the line to the radiator not in the pan). I've only hit 225ish a few times with that same gauge.

 

I think I have narrowed it down to either my epc or tcc solenoid. I plan to install a rebuilt valve body with new solenoids in the near future. Found some info on an explorer forum and believe this is my best bet. Any thoughts on that? (obviously I am learning on the fly here about transmissions so I'll take any input I can get)