I am wondering about the fan, myself. I have certainly heard a lot of noise as part of the symptoms. Is it possible that I am misinterpreting the noise as over-revving (low gear)? To be honest, when this occurs, I naturally panic a bit and pull over to avoid blowing something up. The next time this occurs, I will observe the revs and temps and proceed as if nothing bad is happening. Based on what I've learned about the shift points in T/H from reading a reference on Diesel Place, I think I can ignore the racket and stay analytical for a while. If the revs get above 2800, then the tranny isn't shifting up.
As for the dealers, I've tried the one we bought Sierra from. They know nothing. The closest dealer to us doesn't either, but the adviser is at least willing to investigate and his dad has a 3500 Silverado, so he's interested. After I researched tire pressures, he found an article in the GM service network that backed up my findings. Previously, the "guys" just did what they always did, which was wrong
How does one find a dealer that has expertise?