I'm sorry, I thought this camera was supposed to have High Speed Sync?
It's not the camera, neither the canon or other flashes, it's the Pocket Wizard triggering system for wireless flash. Use anything else and the problem is gone.
I played with a 5DIII this afternoon.
- sync maxed at 160
- sync with pw maxed at 160
- sync with Pixel Kings maxed at 160
All with flash on hotshoe.
It is just to easy to point at pw as the source of all problems and not look elsewhere