There are ADs against many mags, some of them dealing with
impulse failures. Get the mechanic to check for any that apply to this
mag.
Corrosion and sludging often cause sticky impulse couplings.
Neither are good, especially the corrosion, which can cause spring
breakage and power loss. Mags on infrequently-flown airplanes, or those
mags that haven't been off for a long time, definitely need checking.

Dan

Dan