Geometry Editor selects random polygons

When trying to create a weight map, I like to select polygon loops with the Geometry Editor, then switch to the Weight Brush Tool to enter a precise value. However, I've run into problems where the Geometry Editor just picks random polys. If I use it as a brush, it will select polygons that aren't close to the brush, often picking things veritcally when I brush horizontally (and vice-versa), always away from where I'm brushing. If I just select a region, it usually produces the weird results shown below. 

Annoyingly, I don't have this problem with unrigged geometry…but when I use the Transfer tool to rig an item, it clears out the simulation data and my dForce Weight Map.

I can probably get away with painting this, since the selection I want is simple and doesn't involve a complicated gradient, but it's happened to me several times and I'd like to fix the problem.