So I have a old Chevy tbi 350 with a 700r4 automatic trans.

If I floor it the car hooks great and doesn’t seem to slip. But sometimes it feels as if while it’s shifting from 1st to 2nd, or 2nd to third like it pops into a neutral like state. Then the engine just revs up then shifts into next gear.

Should mention if I drive the car slowly it doesn’t do this and shifts normally .

Thoughts?