Thanks for all your suggestions. After going through extensive testing, I felt it had to be a problem with the lens itself so I sent it to Canon in late April. It was returned early last week, and the difference is phenomenal. I'm finally getting the results I should have gotten in the first place: very sharp images and a significant increase in the number of keepers.
According to Canon, "It was found that the adjustment of the circuit board was incorrect causing inaccurate auto focus."
Before the adjustment, I was getting about 10 percent keepers. Now, I'm getting about 70 percent. The 30 percent I end up deleting are mostly repeats and/or poor technique (excitement causing vibration) with probably 30 percent of those due to an actual miss in focus. This is probably more in line with how the lens should perform.