I also have the problem, that the strength of my shooter coil seems to be very inconsistent. Sometimes the ball goes easily all the way to the left orbit, sometimes it barely reaches the top lanes. I have the rev. C shooter lane, so the ball at least almost always makes it over the hump.
I looked a bit closer at my coil and it seems that the coil is too short to really be fixed in place by the coil retaining bracket. My coil reads 090-5023-00 instead of 090-5023-01 in the manual, but I don´t know wether that makes a difference. The fact that the coil is not completely fixed in place could cause the inconsistency, because the plunger sometimes might scratch the retaining bracket and thus loosing force. I hope my English is good enough to make it clear what I´m trying to say.
I will try an extra washer between the coil and the retaining bracket to see if that solves the problem. I´ll report the result.

the metal washer is there, but it´s not able to secure the coil, so I switched the coil tube to this one:
sleeve_70673.jpg
Now at least the coil stays securely in place - still I´m having balls that barely make it up the shooter lane.
I´ve looked really close at how the ball goes off the plunger and up the lane and I´m getting the impression, that sometimes the ball hits the first "ring" of the shooter lane, which of course slows it down substantially.
This could be caused by some misalignment of the coil and shooter lane and I´ll have to check this a little more on my GnR. Just wanted to mention it, because it might be the actual cause for some of you folks, too.