The coil itself cannot be "variable". It could , lose power over time if internal shorts happen and the coil itself then is modified, but it cannot vary back and forth between strong and less strong. If all your coils would act that way, then you could start thinking there's an issue with the power and voltage dips.
There are many factors that can affect the ball, the first ones being the plunger tip's condition, and where exactly it hits on the ball. Then as soon as the ball launches, what happens? does it hit the sides, deviate when it enters a track on the playfield or does it follow a metal rail that could be misaligned and it sends the ball not exactly where it should.. This is exactly what happened on my fish tales. The plunger didn't hit the ball dead center, the tip is a little worn as well and as soon as the ball launches , there's a guide on the playfield, but sometimes the ball bounces off that guide and goes crooked. Right after that it hits a metal rail, that would send the ball towards the bottom too much. After re-aligning the plunger as best as I could (im about to order a replacement bracket because this one was welded back together, crooked), wasting lots of time on the guide for nothing, since every launched ball is different, and re-aligning the metal rail, I do not have that feeling that the coil varies anymore.. The ball now always reaches the track it should.. Speed might vary at times, but that now depends on how it hit the other rail , then the spinner.. etc
As I was suggested, most smart phones have slow motion modes nowadays, which helps to see what happens with the ball when it launches , and where it might lose it's speed. Just changing the coil most likely won't help , but since you most likely need to remove the bracket to change the coil, you might end up re-aligning it better than it was, without noticing it happened.