For troubleshooting purposes, start with a thorough cleaning, make sure you clean the chamber with a brush and good solvent.  Once clean, swab out the chamber with denatured alcohol to remove any solvent or lubricant that may cause casings to stick or drag out of the chamber.  Once everything is clean, lubricate the 7 lugs of the bolt, and rails on the bolt carrier with a quality lubricant like Slip2000 "EWL" Extreme Weapons Lube, so that everything is wet and looks like it's glissening like a glazed donut.

From the process of elimination you reference that you believe it's the bolt carrier (not the bolt).  As for what could be wrong with the bolt carrier, most likely a loose carrier key (gas key), or broken gas key screw.  Generally when we have seen the broken gas key screw, you can see a jetting of carbon coming out from under the carrier key at a 45-degree downward/rearward angle.  

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Have you tried just checking the stripped bolt carrier (No bolt, cam, firing pin) into the upper receiver to see that it doesn't bind, as it should free float down the upper receiver with no resistance, though you might feel a slight something when the gas tube mates inside the carrier key (gas key)?

Another thing to consider is the bottom of the bolt carrier, where it mates with the hammer as it recocks it.  Generically there are two types of bolt carriers when it comes to the bottom where the hammer makes contact as it gets recocked, the shrouded covers the firing pin, and the unshrouded has been cut away so the firing pin exposed. If you have an unshrouded bolt carrier, where you have the relief cut that looks like a ramp and exposes the firing pin, this may allow the bolt carrier to drag as it recocks the hammer, especially if the machining is off.

   

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Then three things to check.
First the the key to make sure it does not have a major leak that heat is causing it to be worse.  CLP around the base of the key to the top of the carrier and with the bolt and cam back in place and the bolt held inwards, use around 100lbs of compressed air in the front of the key to check for leaks of the key to top of carrier.  If there is a leak, then the gas key will need to be reinstalled with new key bolts (do not reuse the one ones, and make sure to clean up any burs on the mating surfaces that are not allowing a clean seal of the two surfaces.

Second is take a good look at the back of the trigger and disco tail for dents/white lines of the hammer tail tagging the two parts hard from the hammer wedging between the two when the trigger is retained back. Also, over cock the hammer to make sure that the hammer tail does not make contact to the top of the disco hook section.    If if just dents in the tails of the trigger/disco tail from hammer wedging, then normal .030" of metal removed for the end of the trigger tail will resolve such problems (i.e. top of hammer contact pad too tall at the top of radus in a none L top cut hammer).

The last if if the firing pin retainer pin is being bent to hell.  Here, ideally the trigger and disco will retain the hammer low enough that the firing pin retainer will not catch the Semi auto firing pin in a un-shrouded carrier (outer edge of FP stop collar should be flush with ramp)/ but some triggers are too short from the pin channel to the front sear, and will cause the hammer to be retained too high.  So in the causes of an L top of FP contact pad on a hammer, the bottom of L cut may need to be radius instead.

Simply, photo or type of hammer, carrier and FP (to confirm is not a larger stop collar M-16 OD, if used in a un-shrouded carrier) would be handy to trouble shooting, if the problem is not just a gas leak between the key and carrier that heat is not making the problem worse.