Hopefully the flush didn't dislodge something and move it to another part of the trans. thus causing the issue. I know a lot of times if the trans is flushed it can cause issues. I would think that taking off on a hill the converter is out of the picture as far as locking since (and I can be wrong here) the converter only locks in 3rd and OD. If you have having issues from a dead stop like that I would think more of a forward band application issue. With you letting off the throttle a bit then getting back into it, it makes me think that whatever is sticking and not allowing full engagement finally catches when you get back on it. Pressure drop when off the throttle then a rise when you get back on it. Kinda like kicking it into place.
That's just me thinking outloud though. I am by no means a 4T65E expert. Just seen a lot of strange things, lol...
How does the fluid look? Nice and red? Is it full? I would check the level and be sure they filled it properly. Just get it fully hot, park on a level surface and leave it running to check it.
