Saw it and it looks nice. Couple of thoughts on the firing after letting up
on the trigger.
1) Insufficient gas pressure.
2) Sear is not rising fully.
3) Sear is worn, rounded and/or sear spring is weak.
It seems like the op rod "wants" to catch on the sear and then it lets go.
Take the small square plate off the bottom of the trigger group (just in front of the trigger area). While the plate is off, get the sear and spring out and get in there with some solvent to make sure there is no gunk in there to impede the sear rising when you let up on the trigger. Make sure the sear does not appear worn. Try another sear/spring combo if you want to; they should pretty much be interchangeable unless you have a late sear as a replacement (they are not cut for the sear safety and can't be used with the sear safety in place.)
Type 96 LMG