Just to add to the posts already made, I would suggest, not saying you have to do it, running a couple boxes of WWB 230FMJs through it with the gun well-lubed. As was stated, most modern 1911s are pretty tight from the factory. I always do that to help smooth the gun out and remove any tight spots.Before any of you purists attack, I know it's one of the annointed DWs but, seeing as how my oldest son just purchased an Ed Brown that went through the 'New,tight-gun' syndrome, it just shows that any new firearm can have these problems.
As for an Ambi Thumb-Safety, I'm a lefty myself. I've had a Cylinder&Slide 'High-Grip' Ambi on my Rock for over 14,000 rounds, still tight,still positive. As with many things, the devil is in the details, it has to be fitted correctly. That said, Kimbers' Ambi with the extended Hammer-pin is probably the best
Ambi out there. It's based pretty closely on the old Kings' Gunworks design and won't come apart, very positive. I had one on my Pro Carry II and it worked very well.
Ambi's, or even one-sided Thumb-Safeties that break or come loose, were not installed correctly to begin with. If you go to the Dremel Room you'll find some photos I posted of both how a Thumb-Safety works and what needs to be fitted. Just remember to go slow and file a little bit at a time lightly, especially on the ledge (Sear Engagement area).
Here's a link to the photos;
http://www.cotep.org/forum/showthread.php?t=646