Just finished going through the engine and transmission in my 08 G8. A friend and I rebuilt the transmission, it had 0 issues before we rebuilt it. The only reason I did was because it had 149k miles on it. The rebuild included a new GM friction kit, updated pump and cover,Sonnax O ring end plug kit,boost valve,1-2-3-4 apply piston, 4-5-6 apply piston kit and Hughes 2500 stall. I put everything together and followed a tuning guide and it worked great. I ended up with a lifter failure and went to a bigger cam. I had the car dyno tuned and the tuner did it in Speed Density (not my idea he said it was required because of inconsistent MAF which ended up being a missing plug on valley cover that I later found) and after the tune thats where I ran into this issue. Under around 50% throttle 1-2 shift is great barks the tires, same with 2-3 shift BUT I normally end up easing off the pedal around this point and the trans downshifts back to 2nd and will hold it for 3-5 seconds then shift back to 3rd. All other gears are fine and operates well otherwise. I have attached my current tune. I have 3.45 gears and 285/35/19 tires. Any advice is greatly appreciated. thanks