Back in Zeb Wells run all inconsistencies in Magik's actions could be explained by "she's doing it to gain Power and screw with people's heads in a Long Con."
With Bendis, her fluctuating power levels seem to come from "lazy writer's syndrome", since she could usually instantly solve any problem that would be a challenge to other members of the team (like Thor in the Avengers, Silver Surfer in Defenders etc, a major power level mix is challenging to write plots for... Someone who can deal with Elder Gods, Cytorrak, Phoenix, and Dormammu isn't well suited to battle Purifiers along with the O5.)