I get to the point where I'm supposed to jump down the shaft and land on some little platform down below. I do this but I can't land! I keep floating up and down. I've tried moving all over, up down left and right and I'm just bobbing like a cork in a pond.

Sound like your using the Noclip cheat hehe ;) Whats are your specs?

I had the same problem with no cheats involved. My usual solution was to load and try to land on the first drop attempt, but on a playthrough I found what makes the biggest difference is lining up directly with the platform. I think the platform blocks the wind. The tunnel is curved, though.
I suspect I've developed some superstitions about how to obtain that sweet spot for the drop. Among them:
Don't worry about the opponent. Making the drop with guns blazing and feet well-placed will prove more important, and you are going to lose health. (Or energy and armor, as the case may be.)
Don't hug the wall on the way down, but feel free to hug the proper wall as you get close. The wall seems to slow your descent, which is bad.
Quick Save before you leap. It makes all the difference.

I just automatically explode every time i enter the room.