When I cut the converter open I will get some pics to explain what I think is going on. I suspect the stator in this unit is the one I refuse to use because of this kind of operation. The statos restricts flow to the pump to increase the stall. When the stator freewheels it will try to make the converter more efficient. In this case I suspect the stator with a part number ending in 086 is being used and that stator has very hard pitched but very spaced out fins. it allows plenty of flow but the hard pitch makes it very inefficient. It is done to increase stall. When that particular stator freewheels at a high speed it will begin to restrict the flow in a way it shouldn't. GM discontinued use of that stator in 2004 and replaced it with the best stator they have ever built.
I just wish they were easy to find. In a few years they should be.
Alan