New problems after L87 replacement

[Todd G 22HC]

I apologize if I missed this topic. I have searched high and low. I, like many others, had to have my 6.2 replaced after having it die while on the Interstate. Fun. It took a little over two months to get it back from my local dealer. Right away I noticed an issue that was not present previously. At idle and between approx 1200-1500 RPM's (usually only after warming up, but possibly when cold as well) I get something between a shudder and full out battle between the engine and transmission. The same dealer lamed it on poor fuel and said it was an issue with detonation. So the next 5 fill ups were with 91/93 octane fuel (depending on what I could find) and the 91 was always ethanol free. NO IMPROVEMENT. I reset the system every way the internet tells you. It reset, but not what I need to be reset - assuming that's the problem at all. The worst part of this is that I never had this issue previously and they aren't willing to do anything with it except charge more time for diagnostics. Part of me thinks they screwed something up, although I am aware it could be an odd coincidence. I would appreciate any insight or tips beyond the basic things I have found. I have a basic scan tool which verifies that there are no codes. Pretty frustrated. New tires and new engine but she's a driveway queen aside from the times I decide to roll the dice. TIA.

[JimCost2014]

Not the same truck, but I guess a similar problem. Had a pushrod break in my 2016, engine replaced and had so many problems the truck ended up being a buy back from GM. One of the issues I did experience was dying while driving at freeway speeds. I cannot be sure but am confident they did not do the relearn correctly when installing the new engine causing a communication problem between the truck and new engine.

[silveradosid]

i would contact GM and start a case with them, hopefully they will instruct the dealer to do something about it