Bug with smoke?

I am just getting started using Blender (2.64) and am following along with Andrew Price / BlenderGuru’s flamethrower tutorial.

I am seeing an odd ringing artifact near the edge of planes (set for collision, static) in my smoke simulation’s domain. It seems to behave as if there is a wall there that most of the smoke bounces off of. The core of the smoke (where the bulk of the fuel would be if this was an actual flamethrower) seems to go through it, and even bounce off of the wall-apparent from the other side.

What am I doing wrong? How can I fix this?

Thank you for your time and patience.