Aperture Brush-in is "off" while working, correct on export?

Question

I have a photo where the subject needs to be "lightened" as she was in shadows, while the background needs to remain at the current exposure. I've done this on several other photos in the set using a Curves block, brushing over the subject entirely, and adjusting the exposure on the subject that way.
On this picture, however, I notice that if I am brushing in WYSIWYG mode, when I brush something "on", the brushed area "shifts" over a few pixels. It is very possible that this is also happening on my other pictures, but due to the level of adjustment I'm doing with this particular photo it's most apparent here, ending up with a dark/light halo all around her.
This is very hard to deal with, as it then looks like my brushes are including too much or too little. If I use the "Color Overlay" the brush appears to match up perfectly, but, again, if I switch from the perfectly-matched overlay into WYSIWYG mode (or deselect the brush-in block altogether), what is shown on screen is again "shifted" but just in the brushed-in areas. However, if I export the photo to JPEG, it uses what I see with "Color Overlay" and the output is perfect. I also notice that areas where I've used the brush-smoothing tool to smooth out the transition from brushed to not brushed show up as 100% brushed on in the "zoomed out" on-screen preview, and at any of the predefined views (Command-+) they draw correctly.
As it stands, I can't use the Aperture view of this photo to show the client, as it looks like she's been cut out by a kindergartner and pasted in again. I also can't give this picture my standard level of attention as each adjustment requires making an export-check-try again round-trip to get it right!
Are these graphics glitches normal? I've tried this on my 27" iMac as well as my year-old 15" MacBook Pro, and while the specific offsets differ, I am seeing the same effect.
I've tried deleting and regenerating the preview, but it remains just as ghastly after regenerating it.
Help!

Answer 1

Thanks for the suggestions.
I've been brushing in at 100, 200, and 300%; this is the only way for my not-fit-for-surgery hands to get a pixel-perfect mask brushed on! Yes, the worst visual glitches show up when the images is just "fit to screen" (when the "smoothed" area all turns to 100% brushed on).
My system's the top-of-the-line (this April) 27" iMac i7 with Radeon 4850 graphics and 8GB of RAM. I suppose I could be over-taxing it with a brushed-on curves block, but that seems like a lot of system for such a small adjustment.
That having been said, I'll try doing the same brushes on less system-taxing adjustments like dodge/burn instead of a full Curves and see if that's the cause (I can't do what I'm trying to do with these pictures with a straight dodge/burn adjustment, though, so I'll have to go back to using the Curves and just live with Aperture not being WYSIWYG for it, or try out Lightroom to see if that can handle a Curves adjustment better even though I really don't like the rest of the Lightroom interface or workflow!)
Again, thanks for the suggestions, it at least gives me something to try!