After around 1000 rounds or more of flawless firing in my Custom TLE/RL II, I had several rounds fail to fire today. The primers were dented, but not as much as they should be. Does the mainspring need replacing?

Absolutely not, if it's the factory mainspring.  Unless you or someone else cut the mainspring, and I mean by more than a couple coils, it's not that.  The mainspring is designed to last many tens of thousands of rounds, and is quite a bit more powerful than it needs to be.

Anyway, you might have issues with your firing pin safety.  I would recommend going to a knowledgable smith, or calling Kimber.  

Of course, it could always be ammo related too.  I just had two misfires in a row on a reliable gun where the hammer dropped all the way.  It was a Springfield with no mechanical issues or a firing pin safety.  Neither primer had an indentation, and the gun was in perfect mechanical shape.

I had the same happen with a Glock during a class.  Of about 500 rounds fired, I probably had 70 failure to fires.  It was all bad primers in brand new LE Federal ammo.

Quoted: Absolutely not, if it's the factory mainspring.  Unless you or someone else cut the mainspring, and I mean by more than a couple coils, it's not that.  The mainspring is designed to last many tens of thousands of rounds, and is quite a bit more powerful than it needs to be. Anyway, you might have issues with your firing pin safety.  I would recommend going to a knowledgable smith, or calling Kimber.   Of course, it could always be ammo related too.  I just had two misfires in a row on a reliable gun where the hammer dropped all the way.  It was a Springfield with no mechanical issues or a firing pin safety.  Neither primer had an indentation, and the gun was in perfect mechanical shape. I had the same happen with a Glock during a class.  Of about 500 rounds fired, I probably had 70 failure to fires.  It was all bad primers in brand new LE Federal ammo.

Thanks for the info. I don't think it was the primers because after dropping the hammer on the same round several times failed to ignite it, I put the same bullet in my girlfriend's Colt and it went off the first time. Is replacing the firing pin (with a non-Kimber  one) a drop in replacement, or does it require fitting? I'd take it to a smith, but don't have any reputable ones locally.

Call Kimber, they will talk you through it and either figure it out or fix it.

Drop in a Wilson series 70 firing pin.  It bypasses the series 2 safety and will most likely solve your problems.

Quoted: Drop in a Wilson series 70 firing pin.  It bypasses the series 2 safety and will most likely solve your problems.

Installed one yesterday. Thanks everyone.