might have missed it but did you clean the gas piston and plug internally?
to do this thoroughly , use brake clean and drill bits that "just fit".
Google M14 gas piston cleaning tools and you will see what I mean. The tools online won't work though as they are sized for usgi/M1A units.
For the chinese rifles we have to get creative and find the drill bits that fit to do the job of carving out the carbon deposits.
the more we rule things out, eventually we will find the culprit. It's the bolt not moving "occasionally" that has me stumped. If the gas system is clean and checks out for alignment..... I agree with others that the next step is checking the oprod/oprod guide and the stock where the oprod slides. I doubt it's a problem with the spring or spring guide rod ..... but we can't rule it out just yet.
Worse case scenario is that something is malfunctioning at the rear of the bolt as it relates to the bolt tail/firing pin tang and safety bridge...... for this diagnoses you will need a rifle smith such as M14medic to inspect the action.
To add on:
Tell me how you run your rifle please:
1) What kind of lubricant are you using in the receiver, on the bolt lugs, and in the op rod track?
2) Did you thoroughly degrease and then re-lubricate the rifle when you bought it, or did you just go shooting?
3) Have you done the tilt test? (google M14 tilt test). Did it pass?
4) When you wiggle the operating rod guide (pinned to the barrel), does it move side to side, or is it held firm?
5) factory loads or reloads? What grain?
6) when you put on the boyd's stock, did you do any inletting on the upper surface of the stock where the operating rod shelf rests? Can you slide a $5 bill under the heel of the rile when the rifle is assembled, or is the heel sitting hard on the stock?
7) With no rounds in the rifle, if you rack the oprod, is the bolt hold open holding the bolt back every time? With the magazine in the rifle, when the bolt is retracted, can you see if the magazine follower is catching UNDER the bolt hold open and fully lifting the BHO? Some mags (actually most non-GI mags except for early Norinco and Checkmate commercial) have an improperly formed follower that can cause the BHO to jam in the up position and reduce reliability.
8) What brand NM guide rod are you using? Do you still have the chinese op rod spring? Chinese springs are a little longer than USGI and have a smaller inside diameter. Canadian-made guide rods are usually sold a little under-diameter to work with Chinese springs, and also incorporate a longer nose taper to improve reliability with chinese parts. But if you have a US guide rod like a sadlak and run it with a chinese spring, this can prevent the op rod from travelling back more than about 1.5" because the spring binds on the guide rod and prevents operating rod rearward travel. It can be intermittent on some guns. If this happens, the bolt goes back into battery after a short stroke and it looks like the bolt didn't move. It did, just not enough to prevent the spend round from re-loading into the chamber.
