Well I sure read your first post wrong. I thought that the blow by stops when the engine gets warm and then starts again when it cools off. I didn't read it as the engine stops running.
It sounds to me like, somehow, the FSS power wire got switched around with the timing advance wire. If that happens, the FSS will lose power when the fast idle shuts off-when the engine starts to come up to temperature. It's really easy to do that with the GM 6.2/6.5s. Not so much with the 6.9/7.3s because of the way their wires lay out. It's not impossible, just more difficult to do.