Okay
, since you changed the springs and it didn't help the next logical culprit is the gas system.
1.) Gas-Cylinder Lockscrew.
A.) Stupid question but, is the screw on tight and the threads Clean?
B.) Is the valve damaged or cracked?
2.) Gas-Cylinder.
A.) is the Gas-Cylinder cracked at the screw?
B.) Is it oversized, take an empty Remington 7mm magnum case and stick it in after cleaning the Gas-Cylinder, base-first(Secret!!!! The belt on a Rem.7mm mag. measures .530). Now,look in the other end of the Gas-Cylinder where the Op-Rod enters, can you see light around the case? If so, you need another gas-cylinder.
3.) Op-Rod Piston.
A.) It should measure no less than .5256" with a 0-1" Micrometer.
B.) Are any of the rim-edges chipped?
C.) If you answered yes to either query, you need a new piston, Call Columbus Machine, they'll install a new one very reasonably.
4.) Barrel port.
A.) Clean out the cruddy carbon with a toothpick and mike it with a #47 drillbit, should enter the port.
B.) Try again with a #45 drillbit. This is the No-Go gauge for the port. If this bit fits the port is too big, only answer is another barrel.
Good luck and let me know what you find, maybe I can help.
Dave