They are supposed to do that with the stock tune. The pcm unlocks the torque converter when You let off the throttle a little to prevent what it perceives as un wanted engine braking to help with fuel economy. Mine does it alot around 50-60mph. Especially with the cruise on. I also just installed a 2800 stall and now it drives me crazy. I'm hoping it can be eliminated with efi liive. I installed my cam heads longtubes and injectors last weekend and should be getting a dyno tune this weekend. Ill let you know for sure if it can be tuned out.