If you can go to a larger carrier size and still have no leaks, try the next larger size as well. You may not have the largest carrier size yet. Make sure you remove all of the shims when testing the carriers.
The shorter spring will hit harder. Find out the lowest velocity that the gun starts to fire using the spring you want to use. The optimum velocity value for your setup is about 20fps above that lowest value that will allow the valve to cycle. If the velocity is way low, then use a longer bolt spring and do it again. If the long spring gives you a starting velocity that is too high, then start cutting coils off one at a time and test the new velocity numbers until you arrive at a range that works for you. Make sure you put the cut side against the bolt.
The velocity fluctuations can be caused by a carrier that is not a perfect match. The inconsistency in the friction can cause the fluctuations at lower velocities when the friction change is a larger percentage of the differential force compared to a higher velocity setting where the friction change doesn't have as much affect.
Except for the Automag in front, its usually the man behind the equipment that counts.