Using the middle spring, you are probably setting the velocity too close to the lower end of the operating range. This would cause the inconsistency you are seeing. Once the carrier oring is properly broken in, these fluctuations will go away. Until then, you may want to use the short bolt spring. The other option is to cut the long spring down to act as a shorter version of the middle spring. The proper spring allows the ideal velocity setting to make the gun shoot 20fps above the lowest velocity at which the gun stops shooting.
I am trying to chrono at 280 fps. Is that to low for the medium spring?