40d and canon 1.4 extender

I have tried to use the 100-400 lens and the canon extender on my 40d
it worked pretty well on my 30d with the auto focus. the 40d focus is so much

faster with the extender on the camera wont hold focus. so is there a extender out there any one is using that works on the 40d and the 100-400 lens?
like the tamron or the kenko??
thanks for your help

gary