2011 slide intermittent locking back

[BoyGlock]

Searched this issue here and found nothing. Pls. Direct me if you saw same topic. 
My open 2011 has new upper on an old frame. In 1 or 2 times in a hundred or 2hundo rounds the slide locks back to its rearmost position and the barrel and slide are frozen together in a hard lock up. Been doing this for 2k rounds now of .38super major load and could not put my finger on the probable cause, It happens so rarely that I took it as just a break-in issue. The slide/barrel/frame fit is not overly tight. It does this with and without ammo or mag. I dont use shock buff. So Im pretty sure its in the gun. This morning it happened again in dry fire when I racked it from an unloaded start, so I took a hard look see and found I could release the slide if I push down a bit on the comp/muzzle. And I could lock it at will if I rack the slide back to its rearmost and push the muzzle up. Gun is otherwise reliable and accurate but its not how a gun should shoot in matches. 
Can you guys point me where to look in its innards? Tia

[sauza45]

Hard to tell without seeing it but I would look at the guide rod and recoil spring assembly. Try removing the guide rod and see if it still locks up. If it doesn't then take a hard look at the recoil assembly

[zzt]

Also check for barrel springing.

[caspian guy]

1 hour ago, sauza45 said:

Hard to tell without seeing it but I would look at the guide rod and recoil spring assembly. Try removing the guide rod and see if it still locks up. If it doesn't then take a hard look at the recoil assembly

Yep ink up the top of the guide rod head with a sharpie marker and try cycling it and see if you can see something rubbing.  Also make sure the head of the guiderod is a good fit in the frame.

[N3WWN]

Take a look at the firing pin stop and firing pin spring. 

 

If the firing pin stop is very loose and the firing pin spring is very weak, the firing pin stop can drop down slightly before the firing pin can return to hold it in place.  The firing pin stop can then hang up on the rear of the frame/hammer/etc.

 

I had this happen with my first 1911.  Resolved it by replacing the firing pin spring as well as the firing pin stop with one that I fitted myself to be tighter.

[BoyGlock]

Update. Checked barrela and guiderod for possible binding. Found faint marks:

[BoyGlock]

Put some sharpie on them and dry racked the slide some 50x. Showed no contact or bind. 

[BoyGlock]

Live fired 300 rnds .38s major load. Contact looked like due to vibration or impact due to recoil. Think guiderod head and barrel are clearanced ok?

[BoyGlock]

In 300 rnds had minor misfeed twice where the round did not chamber about 1/4”, nudged it and went in. I suspect the slide locked back then released on its own lessening its force enough to cause the misfeeds.  

[BoyGlock]

Finally found the culprit and removed metal from the back edge upper corners of the bull barrel sleeve where the muzzle end of the slide edge was getting down at its rearmost travel and catch it, completely stopping it to zero movement until I push the comp down a bit and the slide springs back forward. My slide was chopped 3/4” shorter w/c caused this. This happened a few times intermittently. The last few times it did I stopped whatever I was doing during practice and inspected the gun thoroughly and was able to catch the real issue. 
Now the gun runs reliably.