Using a Canon 300 F4.0 IS with a 1.4 ii converter on a 5DM3

After using the Canon 300mm f4.0 IS with the Canon 1.4 ii on my Canon 40D with good results taking pictures of flying ducks (as one example), I find the the same lens combo, using Al-Servo in burst mode, does not produce sharp images on the 5DM3. The f stop range I usually use with this combo is 5.6 and 6.3 in AV mode. Tomorrow I'm going to try the 5Dm3 and the 300 without the 1.4 and in shutter priority.

I've read contradictory reviews about using this lens combo on a 5DM3, but my experience is that it does not work well for fast action in AlSevro.

Cropped image

Anyone have a similar experience, or great results?

Badly oof image.