I have thought about removing the air filter to see if that makes any difference. It is brand new and should not be causing any clogged air path. But it is something to try.
Besides not enough blower motor output, what else can cause this problem? I've tried all of the things listed above. I don't see any Heat Exchanger issues from a pretty thorough inspection with a flashlight from the top and bottom. I have thought about replacing the primary limit switch ... maybe it is opening too early. But the housing where it is mounted does get too hot to keep my hand on it ... so I figured that it was doing its job. From what I can find on the internet it looks like this limit switch should open at 180 F and then close again at 140 F. I have no idea if this is where it actually activates.