I've been in a Land Rover doing such things (alhtough not quite THAT steep!) They are amazing vehicles. Under water (well, up to the top of the hood), ascents, descents... they smoke them all. Very fun to drive on an off road course!
I was wondering if maybe the engine was starving of oil. At such grade of incline the oil would probably be all at the front/rear crankshaft bearing... and being whipped up into a foam by the crank... what do you guys think?