904 Valve Body
A couple of common problems I've read about are the low/reverse servo getting cocked in the bore (there's a problem solver super servo for $39 that fixes this), and cracked clutch apply pistons. Both problems are easy to overlook during a rebuild.
I believe the low/reverse servo problem can cause many different problems, the 2-3 flare being one of them. A cracked/leaking 2-3 piston can definitely cause this problem.