Sounds like a more simplre problem to me.
You are PROBABLY too low on fluid.
Auto trannies can be REAL scary when you don't really know what's going on inside of 'em.
I kept seeing you say you "let the truck sit". It's good to take a break, but,,
Letting the truck sit, isn't doing to much.
What you need to do is,, Put it in "N" rev it up to 1,200 for 4-5 seconds, then let it idle,, put it in Reverse, then Neutral again, then Drive then 2nd, then 1st, then 2nd then Drive, then Neutral, Reverse, then Neutral again. Set your parking brake (leave the truck in neutral),, and check the fluid. It should be completely full, and have NO bubbles.
In Park,, You should read several quarts high, especially if you've removed your Anti-Drain Back Valve.
Move the truck a little,, about 15' reverse, and 15' feet forwards, then put it in Neutral, set the parking brake, and check it again.
It's normal for it to read low if it's a brand new transmission, or you just did a repair, add the nessacary fluid.
There are ALOT of cavities that can eat up the fluid, and a new transmission will take 15-20 minutes to get all the air worked out.
I think thet ATS transmission,, when completely empty will take about 18qts, or a little more. If you put that much and it "appears" high,, your dipstick might be reaching too far into your pan, and giving you a false reading.
If your transmission is low on fluid,, it's going to be going in and out of gear. (Can you pretend it's a manual, and when it's low on fluid,, it's like having the clutch 1/2 way in)
When stopped in gear, it feels like a manual transmission, and somebody is letting out on teh clutch, then pressing it down again, and releases it, back out, releases, back out, and on, and on, untill your transmission fluid has too much bubbles in it, or overheats.
Your pump in the transmission will start sucking air, and will force the air into the transmission fluid. This will heat the fuild up (also there is less fluid to actually heat up), and greatly decrease the holding pressure for thje clutches.
If you drive much with low fluid, the end result will be exactly what you describe,,, burnt fluid, and metal shavings.
My ATS triple Lock if, left "locked" with my mystery switch, will idle locked for about 1-2 seconds before it kills the motor. If I lock it up stopped, it will just shut the truck off just like a manual transmission clutch dump.
Hope you get it fixed.
Merrick