Unique problem with 12.5 upper?

[hkshooter]

I just acquired a new T91A1 upper in 12.5 inch. The FSB is canted, nothing new there.
The lower is a PSA, stock is Magpul MOE on milspec tube, buffer is H2, spring is unknown carbine length. Trigger is Larue MBT.
I'm using a Law Tactical folding mech on the stock. This has never caused an issue with my Colt 11.5 upper so there must be something unique to the T91 bolt group I'm not familiar with.
When I first assembled the upper to the lower and static tested it more than once when I pulled the bolt back there was a binding of some sort that would stick the bolt to the rear. If I wiggled the upper to lower fit a bit the bolt would snap shut. Pull it a few more times, no binding, pulled again, binding. I wasn't sure what was up but it didn't happen more than it did so I test fired it. The first round out of the mag stopped the bolt but it finally went closed on it's own after a second.
The gun ran without fail through the entire mag.
Today I went to zero the red dot, first round out of the mag stopped the bolt. No big deal, I popped the charging handle and it snapped shut. USGI, blue follower aluminum mags, btw.
I worked my way through the mag up to five rounds remaining when the bolt group bound up a little and caused a stoppage, round half way out of the mag. I popped the charging handle again and it snapped shut, fired the rest of the mag.

What I don't know yet is why the bolt group is binding sometimes when the bolt cycles. Is there something about the way the chamfered bolt and the Law Tactical carrier extension work together that isn't compatible?
Or could it be that the Larue MBT hammer is catching the bolt weird or something?
At the moment I'm at a loss, looking for input.

[e292644]

Have you tried mounting the upper to the standard MIL-SPEC AR-15 lower to see if it act the same?

[hkshooter]

It's in the plan. As soon as I recover the rifle I'll give that a try as I have a pistol braced lower doing nothing.