I have a Ultra Carry II mine has the external extractor and I have this same problem sometimes.
First I would like to say I don't think my problem is because of the external extractor. That said this is what I have tried.
The Kimber Mag. has been replaced with a Wilsom Combat ox47 I highly recommend using anything except the Kimber supplied Mags.
I replaced my recoilspring Assy. This something all small bbl. 1911 owners should watch closely and replace as Per. the Kimber Manual. Kimbers Custom Shop will exchange the Assys. for a very low cost.
Make sure the bbl. chamber is clean and make sure your using a lot of lub after each 50 rounds at the range Kimbers need plenty of lub.
I shoot a lot of lower cost 230gr. FMJ Ammo. at range like WWB and Blazer so I check my carry Ammo. and see if it has any problems. It's the real important Ammo. you don't want problems with.
Watch this Todd Jarret Video and notice what he says about holding the 1911.