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Agreed with other's statements that the transmission is problematic. In my experience with this transmission, and a very similar version in my Traverse, there are two common issues.
1. Shift flare, which is a delay between when the transmission lets one gear go and then engages the next. This usually happens between 2nd and 3rd. Simplified, the pump component of the torque converter isn't providing optimal pressure for the shift change, so the trans waits for pressure to get there and leaves you sitting there feeling like you're in neutral for fractions of a second. At the beginning, it only happens at very specific combinations of rpm's, speeds, throttle position, etc. On my Traverse the problem has gotten worse over time and it has become very hard to accelerate without the flare. Feathering the throttle just right before the shift is the only way I can avoid it now. A torque converter replacement is likely a forgone conclusion on the Traverse now.
2. Second issue is the identity crisis that the trans experiences when it doesn't know what it wants to do. Should I stay where I am, upshift, downshift, mens room, ladies room??? The 4 speeds in my Pontiac Grand Prix's had similar issues to a lesser degree. It's like GM took the same design and then threw in 2 more gears to ensure occasional chaos. On both vehicles, I eventually adapted my driving style to the trans (and lightened up my lead foot) and now I almost never have these issues. Only occasional thuds when slowly slowing to a stop up a hill. Older GM trans (25+ years old) used to adapt to me and I drove how I liked. The newer ones increasingly require me to adapt to the trans.
I am very interested in experiencing this engine in the Camaro to see how the experience differs with RWD and a more capable trans.
 

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Yeah, our '16 Traverse had this with 2-3 shift under heavy acceleration or heavy load (up a steep hill) at about 50k miles. All fixed under warranty. It is a violent shake that could only be confused with a bad misfire. I've never exprienced TCC shudder anywhere near that bad.
OP states they can get it to stop by backing off or flooring it. For us, the only thing that would stop it was backing off the throttle. I tried powering through, but that didn't do anything but burn the clutches.
GM ended up partially rebuilding the trans twice, doing the clutch packs and other parts, and cleaning the valve body each time. But each time the problem came back after a few days of driving. Finally they just replaced the valve body on the 3rd try and that worked. They could have just rebuilt the whole thing the first time for less than what they ended up paying the trans shop for the 3 attempts. Didn't have the Traverse for nearly a month while they repeatedly failed to fix it. But GM did rent an Impala for us for the bulk of the time (which led to our eventual purchase).
The shift flare problem started about 5k miles after the repair. Had I known, I would have paid out-of-pocket to have the torque converter replaced while they had the trans out.
 
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