c32 loses power

[Mike400E]

2002 C32. When the engine gets completely heat soaked, usually after 30 minutes or more continuous running, the it feels like it goes to about 1/3 power. It still runs, and there is only the slightest of shudders. There is no bogging, exhaust smoke, or any other symptom I can see. Unless I am under power, I cannot even feel the difference. But as soon as I apply any power at all, there is simply nothing there. (Like an auto Hyundai Accent or worse.) If I am running up hill or accelerating, it will stay in this condition until the load is stopped, then it will go back to full power within a few seconds to a minute or two. It will cycle back and forth, only losing power ocassionally, and only for a minute or two until it has a chance to completely cool down.

Any ideas? This is a weird one!