Since it is getting the "reduced power" message on the panel, it sounds like the computer is sensing a problem and going into "limp home" mode where it uses a preset map that lets the motor operate, but not at full power or efficiency.
That could be the sign of an actual problem, or since you say it goes away, it is more likely a sensor overheating, and then it starts working again after cooling off. I'm not familiar enough with the 8.1 to know which sensors tend to do this on that motor, even though I have an 8.1 in my rig.
On other vehicles it's quite often caused by a crankshaft position sensor or a camshaft sensor going bad. Some will throw a CEL code, some don't.
They really need to find someone familiar with GM V-8's, especially the 8.1, and see if they can shed some light on which one is likely to go bad.
