It's a very sharp lens, very. There are tricks to using it.
First, don't use a filter, one more piece of glass to go through, esp at 400mm. And some say there are other hypertechnical reasons as well, ? filters don't work with stabilization.
Second, use a tripod, and if there's wind, a weight on the center column hook. Or at least a monopod.
Third, use as fast a shutter speed as possible.
Fourth, use your microadjustment to calibrate it to your camera body.
Fifth, keep the tension ring relatively tight, so that when it hangs down in your hand, the barrel doesn't extend on its own with gravity and whack the stop mechanism.
Last, I use TF Camera Repair in NJ rather than Canon for overhauls. They test to higher standards than does Canon.
But most of all, it helps if you've played the trombone in high school.