If you are having autofocus problems on the 150-600, your problem might be camera configuration.
If you go into the camera setup menus, under Autofocus/Drive, there is an option for Lens drive when AF impossible... You can set it to "Continue focus search" or to "Stop focus search". If you set it to "Stop focus search" the lens will give up very easily and AF will not work well, particularly when going between near and distant focus. If you set it to "Continue focus search" it works far better.
(At least it does on a 60D)