WGJ
I had the same problem on my rebuilt tranny. 1 to 2 was a neck snapper and 2 to 3 would flare from 100 rpms to 1500 rpms 3 to 4 was fine. The shop found out that is was the resistor behind the battery. I was also cooking the high clutch package at the same time. warranty covered it.
I would take it back and have them check the high clutch and replace the resistor. Sorry about the bad news.
Steve