I am facing similar issues. It seems to be my "d3d_texbmaps" and d3d_texskins" that are stacking up with each level load.

I've created a smaller game which emulates the functions of my game in a hope to find out where the leak is coming from. So far no luck...

Each time I add a function to the smaller game to see how the memory gets handled, and so far the memory is reset on each level load. There is no stacking. However, if I keep adding functionality similar to my (big) game, eventually I'll either have a working game, or I'll discover exactly what's causing the problem.

Level_Load is "supposed" to clear out the memory, but there must be some limitation, or some loophole that allows memory to build up between loads. I will definitely follow up if I find the source of my leak.

UPDATE:
Actually, I can reproduce this memory drain by loading a .wmb file into my level. It increases the overall RAM usage each time I load it, even if I purge and remove it from the level, and even after I load a new level.