OK. I think I'm figuring it out. I pulled the air divider off the heat box. When I stick my hand in the box, the bottom layer of air coming to the divider is definitely cooler than the top layer of air. I guess when the air hits the divider, all of the top air gets pushed to the far left of the divider, and the bottom layer gets forced to the right side of the divider. So, what's causing the cooler bottom layer of air? Based on this excellent thread:
I believe I've got an idea. It must be 1 of 2 options. 1)My blend gate does not seal off completely at the bottom, and is allowing a smaller stream of air to bypass the heater. For whatever reason (maybe different velocities of the two air streams?) the air is not blending to one temp before reaching the divider. Now, I can hear the blend gate hitting the bottom of the box when I put it to warm. It sounds like plastic hitting plastic. Should the blend gate have some sort of foam around it to make a tighter seal?
Possible problem 2) Can a portion of the heater core clog up? Maybe one side is staying cool? Actually, as I'm typing this and thinking of it, that seems to be the more logical choice, since I hear the blend gate closing against the box. Both heater hoses get hot, so water is going in and out, but maybe its only circulating through one side of the core?