The tranny acts like it wants to slip or shift late, but this is only under load. 1-2 shift is very precise and predictable, but 2-3 is kinda -eh. I took it back the first time it happened and the shop tore it back down and installed Z-pack clutches and enlarged a few holes to get line pressure up. I was told to raise the rev. limiter up(poss. hitting the limiter on 2-3) and this would cure the prob. They said this is common when going with a higher stall (flashes 3200). I did this and it worked for a few miles, but now the problem is back. I use a HPPIII it worked great for the stock trans, but after the rebuild I returned all shift points back to stock. Seemed to me that if the 2-3 shift is set to shift later shouldn't 1-2 do the same. We used a very reputable shop, and they have not had any others returned.

 

Ask them what they drilled the 3rd feed hole to. About 1% of 4L60-Es don't like oversized 3rd feed hole.
There is a way around it.