The fans jitter? Like make a noise/vibrate? If thats the case, then they either might be mounted wrong, or bad. However for the fans spining for awhile after the power goes off, thats a great thing. That is an indication that the fan is low friction (i.e. low power consumption, because it has to do less to do the same, lol). Just check your fans, and see if they are snugly screwed down. If they are jittering, then maybe that should help.
Hope this helps.