So, just to update this thread. The converter not unlocking when going into 2nd (which is what I've narrowed it down to be) had come back.
I AGAIN dumped the fluid and put in 6L of Mercon V (the other half of the case I bought) and noticed an immediate improvement; it started unlocking again.
Drove it for about two weeks, then today, dumped it AGAIN, put another 6L of Mercon V in it and it seems even better.
So I'm going to go out on a limb here and say that the tranny likes Mercon V more than it does the (non Mercon V) Mobil 1 fluid.
To elaborate on the issue, the shift pattern of the truck, under moderate acceleration is this:
Converter unlocked---->Accelerate to over 4,000RPM---->Lock Converter---->Accelerate to over 4,000RPM---->Unlock converter as it goes into 2nd gear---->Accelerate to over 4,000RPM---->Lock Converter.....etc. It does this through all 4 gears.
Now the PROBLEM was that ONLY going into 2nd gear, it would sometimes NOT unlock the converter. This caused a rather large drop in RPM and dragged the engine out of its power-band. Where it would then labour back up INTO its power-band and resume (correctly) through the rest of the gears, or until I let off the pedal.
This occurrence seemed more frequent with the M1 fluid in there. As I continue to add Mercon V, it seems to be disappearing (hopefully completely).
I have no idea what the cause is. No codes or anything, tranny shifts fine otherwise, and this ONLY happens when you go to do some rather hard acceleration.
I will notify as to whether this has fixed it permanently or not......