Magnetars are also known as Soft Gamma Repeaters (SGR's), not as violent as Gamma-Ray Bursters. Unlike those exotic objects, SGR's are known to be periodic in their fluctuations, but within defined limits. Like pulsars, their magnetic fields are so strong that they induce starquakes as the fast-spinning star rotates.
They are unlike pulsars, however, in that the periods of magnetars are slowing down rather than constant. They are also associated with massive star clusters so are at low galactic latitudes, whereas pulsars are found in other places such as Globular Clusters. We don't know what is slowing the rotation rates of magnetars down, so they remain jealously guarding their secrets.