Thanks for all the replies, guys!
rampy - I did think about that, because I am using one of those true-flat monitors that has the curvature on the inside of the tube. That was why I tried it on my W-G, and it did show some improvement. I wonder if my problems weren't a combination of a couple of things.
JMD - Yeah, I didn't word my explanation very clearly. I realize that the flashing is what registers the shots, but it seemed that when the shots were very rapid, causing the screen to flash very rapidly, is when the problems were at their worst. That's what I meant by the flashing causing the mis-targeting.
Dave - I guess I didn't set my res low enough.... The lowest I tried was 1024x768@75Hz. By setting it at 800x600@75Hz, everything has improved greatly! The Act-Labs site says the gun supports resolutions up to 2048 x 1940, but I had difficulty with 1024x768. I'm sorry I missed your previous post about those settings.
Thanks again, everyone!