[Terwin]

If the players understand the rules that control magic, then it is not a mysterious and cosmic force.

If magic is reliable and consistent, then it is not a mysterious force.

If you want magic to be mysterious, then you need to have some motive behind it that influences the results of just about every spell, but the goals and intent of this force must not be known or easily identified.

One approach might be to think of spells as opening a portal that allow some cosmic being to reach through and affect the world according to it's own whim, but also puts a post-it note on the door-frame saying 'please do X'

Now this cosmic being may have specific goals it is pursuing(preferably ones beyond the understanding of the PCs and only the oldest and wisest of sages can even get a glimmering of which way it is trying to move), or it may just be bored and capricious(imagine Q from ST:TNG but everyone with magery can tap him on the shoulder and make a request)

This is the sort of approach that makes magic a force with a will of it's own.
You can make a request, bu regardless of how well you cast the spell, the odds of getting what you want are fifty-fifty at best.

If you want magic to be a little more controllable, you could assume that magic is drawing power from some cosmic entity, you might even make up a hit-location chart for that entity, and each location has an impact on the spell effect.
But the stronger the spell, the more likely the entity is to notice and decide to intervene(perhaps it likes the caster or the caster's cause, or perhaps it dislikes having it's power stolen and will exact vengeance, perhaps there is more than one entity and they cover a whole spectrum)

The problem is, to make magic mysterious you must remove much of the control from the hands of the spell caster. And you can't even explain the details of what and why, you can only inform them that magic is very fickle and any time a spell caster gets exactly what they asked for, they were very lucky.

Of course none of these options are conducive to flinging fireballs at orcs on a regular basis...