Delayed and bad shifting

2005 Duramax. Havent tuned anything in the tcm, but after flashing the ecm with new tuning for the truck, all the shifts cause the truck to cut out and drop my rpms for a second before the shift, then rpms come up a bit, then rpm lower and it shifts into gear. This is also while driving as light on the throttle as can be. What causes this and how do I correct it?

How many miles have you put on it since tuning it?

As Brayden commented how far have you driven since doing the tuning? The Duramax does require some distance to be driven and multiple shifts along with key cycles to relearn.

If you have done quite a few miles then it could be something in the ECM tuning which has altered the trans. The trans will alter the shifting based on parameters and torque calculations which are provided from the ECM. Altering certain parameters too far will cause shifting issues in the trans.

If you can put the original file back in the ECM and it starts to shift better, then that would suggest it is a parameter in the ecm which is causing it.

There is a great worked example of Duramax tuning in the Practical diesel tuning course. It also touches on how some tables can influence the trans.

https://www.hpacademy.com/courses/practical-diesel-tuning/