Hi everyone

Have a problem with my Vortec 5.7 -4l60e.
It has started to “stumble” Or “Hacks” (mike to violently) sometimes when driving. It almost feels Like when you hit the rev. limiter and it just shuts of the ignition.
but it feels when driving like it’s in the gearbox.
When accelerating half-full throttle it never happens, neither below 45mph, only on steady speed mostly between 50-70mph, not above that. Most frequently when letting off the gas after a small climb.
No error codes or CEL or anything.
How does the gearbox gets signal for shifting/lock up?

would be super greatful for any suggestions and help to troubleshoot.