Your gas port is too small, and I stated that without know the gas port diameter, just from your description of the function of the rifle.
A .308AR gas gun (firing .308 Winchester ammo) 20" rifle-gas barrel with a 0.750" gas block journal size needs a gas port diameter in the range of 0..093"~0.096".
Your gun is never going to run right, until you drill that to something between 0.093"~0.096". All barrels are not made the same - as you state, you have a 4-groove - so that's why there's a range, and not an exact diameter. Something inside of that range will make your gun run, as fas as gas system goes. You need to address the recoil spring, and the buffer weight, because this is a balanced system - they BOTH have to work together.
All other functioning issues, after solving these items, will NOT be a gas system issue, or a recoil system issue.