I will attempt to impart wisdom.
Full auto issues should first be addressed through the fire control group. There is a slender piece of metal in the FCG that holds the valve open until the hammer is cocked. This piece interacts with the valve knocker.
Over time, this slender piece of metal will cause the valve knocker to split on the thinnest area surrounding its pin by creating a camming action between it and the force of the magazine valve pressing on the valve knocker.
In essence, I would say the full auto issues can be traced to the valve knocker. Check to see if its either broken or loose.
As a precautionary measure, I would suggest removing the slender piece of metal referenced in an earlier paragraph, as it provides an irrelevant feature at the cost of severely reduced parts durability. That feature is the strength of the recoil. The recoil without that particular slender piece of metal isn't reduced too severely, but it is discernable. This is the only negative side effect of removing that slender piece of metal.
The result of removing the slender piece of metal, and if necessary repairing/replacing the valve knocker, is a dramatic increase of shot to shot consistency.
Edit: I realized I had failed to mention how exactly a broken valve knocker affects function on full auto.
I've never given much thought on their relationship, or the relationship between issues with full auto and any other part in the gun.
If I had to guess, I would say that the existence of the slender piece of metal messes with the timing of the FCG. It would release the hammer too early, which would cause it to ride forward along with the carrier.
Specific causality aside, I would say that this slender piece of metal, overall, causes most of the known problems with the function and performance of the GBBR.