Other suggestions:
Did you try different slopes? I mean: different steep slopes?
Although, I'm sure you already checked that 'cause you're familiar with the old collision system: Where is the origin of the model?

Did you already look into this:

from the manual:

"USE_AABB P Uses an axis aligned bounding box (AABB) for collision, rather than an oriented bounding box (OBB). The AABB system is faster, but ignores the entity orientation on USE_BOX, treats models and sprites as boxes, and requires a BSP level. See collision for the difference between both systems. "

That's all I can think of and my suggestions could be pretty useless, because you already looked for that and I don't use the new collision systems yet. I still use A6.60 and trace instead of c_trace and such.