Normal driving my trans shifts fine but when WOT it seems from 1-2 gear it shifts as almost its slipping into second. It shifts slowly drops to 4000 rpms then fells like it locks and go. I have monitered spark it seems fine not loosing any.

It feels almost like your in a manual car shifting slowly letting off the clutch then all of a sudden letting it go.

Its on my 99Z71 5.3L 4l60e.

Anything I can tune to eliminate this or is my trans taking a dumb?