Truck ran fine for 11 months then the same thing happened again. While driving down the road the tranny wanted to down shift (going into a stiff wind). It felt like someone just attached a 5 ton weight to the rear end. The engine light stated flashing again.
Shop replaced a fuel injector and spark plug. Next day it did the very same thing.
This time I was told that GM just issued a TSB for this problem. Now they are going to replace the exhaust valve springs. I was told that while waiting for the new parts to come in driving the truck like this won't hurt it.