So funny that I read this thread randomly this afternoon, later in the day showing a buddy what the car could do mine and it did the same thing as noted by the OP. I had turned off all TCs and between shifts of 2-3rd the car shuddered, revved all the way up to red line until I let off the gas and it finally shifted. So i stopped, hit it again from a dig and was able to replicate it. Ended up getting a check manual error and a wrench showed up on the right side of the display. Odd thing too is when I got to my destination (about 2 miles up the road) the car stayed on even when I shut it off and all the displays went dark. Had to turn it on and off a few times to get the car to actually shut down. While in the store my app sent me the error below.
was able to re-create the same issue with TC on as well. I have the 91 ZFG Tune with filter and exhaust. Looks like I might need to run a data log since I can recreate the issue but a little disconcerting.
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