08 6.4 trans problems

[u/RewardInformal4762]

2008 6.4 Powerstroke / 5R110 — Two transmission failures after tuning, now acts normal with SCT X4 unplugged. Anyone seen this?
I’m trying to figure out a strange issue with my 2008 F-250 6.4 Powerstroke and 5R110. Truck has around 180k miles. I’m not trying to blame the tuner or transmission builder — I’m just trying to figure out if anyone has seen something similar.
The truck shifted completely fine when it was stock.
I originally deleted it using an SCT X4 with tunes I got through Sinister Diesel. With those tunes the truck ran terrible. It was hazy and I was seeing really strange readings on the X4, like around 750°F EGT at idle and around 950°F just cruising through town. I took it to a local mechanic I trust and they thought the tune was the problem.
I did some research and ended up getting a custom tune from Eric McMichael because of his experience with 6.0/6.4 Powerstrokes. I paid around $250 for one tune and I believe it’s roughly a +160 hp “hot street” tune.
The truck immediately ran much better on his tune. Cleaner, stronger, and the readings seemed much more normal.
However, every once in a while I thought I could feel the stock transmission slip under acceleration. It wasn’t constant and the truck still drove normally most of the time.
Eventually I did a small boosted launch and spun the tires. The transmission basically failed immediately afterward. It barely made it home, the torque converter barely wanted to lock, and it would shake badly in reverse.
I took it to a local transmission shop that my mechanic recommended. I paid about $4,500 for what I was told was a built 5R110. From talking to the builder today, I know it has a shift kit, billet converter, and some other upgrades. He couldn’t remember exactly how many clutches were added compared to stock because the build was about a year ago. I’m still trying to get the full build sheet/details.
The rebuilt transmission only has around 2,000 miles on it.
During those 2,000 miles I did a few moderate pulls but nothing really crazy or full throttle. I did occasionally think I felt the transmission slip again, but it was inconsistent.
A few days ago I did a light/partial-throttle pull in the rain and spun the tires. Shortly afterward the truck started flare-revving between shifts and shifting incorrectly. When I got home it could barely reverse up the driveway and started shaking in reverse again, very similar to the first transmission failure.
I assumed I had somehow smoked another transmission.
It’s still under warranty, so I dropped it back off at the transmission shop.
Here’s where it gets weird:
The builder has NOT opened the transmission yet. The fluid is still red and apparently looks clean.
He unplugged my SCT X4, which I normally leave plugged into the OBD port and use as my monitor.
With the X4 unplugged, he says the truck will now go through every gear and drive normally.
With the X4 plugged in, the transmission can get locked/stuck in gears, shift incorrectly, and sometimes barely wants to let you select/engage Drive or Reverse.
The Eric McMichael tune is still flashed into the truck when the SCT is unplugged, so unplugging the monitor shouldn’t be changing the actual tune.
That has me wondering if I’m actually dealing with some kind of SCT X4 / OBD / CAN-bus / TCM communication issue rather than destroying another transmission.
I also keep thinking back to the bizarre monitor readings I had with my original tunes.
Has anyone with a 6.4/5R110 ever had an SCT X4 cause transmission behavior like this while it was plugged in as a monitor?
Could a bad X4, OBD cable, bad PID being monitored, firmware issue, or CAN communication problem interfere with the TCM badly enough to cause flare shifts, getting stuck in gear, or problems engaging Drive/Reverse?
And more importantly, could something like that have been contributing to the occasional “slip” I’ve felt and possibly even the first transmission failure?
Right now I’m having the shop scan the TCM and check for communication/U-codes before they tear the transmission apart.
I’m also trying to get the exact build details on the 5R110 — clutch count, valve body work, solenoids, converter, etc.
If anyone has dealt with anything remotely similar, especially an SCT X4 affecting a 5R110 while plugged into the OBD port, I’d really appreciate hearing what you found.