I am trying to figure out an issue with my 5.3/4l80 swap that's driving me nuts!

When the truck is under load at light throttle it surges and bucks between the shift 2nd to 3rd....or so I think. It's odd, when I first drive it it's like it just spins out the converter in second.....I let off and it shifts to third, and it acts that way into 4th sometimes too.

Can you guys explain how to isolate converter or slipping issues with the scanner, please?

Of course the trans is the only part I didn't work on myself...and it's being a bitch!