vibrate and ring setting turns itself off randomly

[purity]

like the title says, the setting "Vibrate and Ring" in the System Settings menu under the Sound category turns itself off randomly about every 2 weeks or so. the only way i find out is because when the phone is in my pocket when im doing something like driving, i cant hear the ringtone and i miss all my calls because i dont get a vibration either. when i check my phone later and notice i have several missed calls the first thing i do now is check the vibrate and ring setting and lo and behold it is unchecked again. i have to check the box again myself to turn it back on and it will stay for maybe a couple of weeks and then it will mysteriously disable itself and i will go thru this process all over again. after having to do this for the past couple of months its gotten beyond annoying. it was especially annoying before i figured out what it was and went a few days with no ringtone vibrations. i dont miss calls if its on because i always feel the vibrations in my pocket so i really need this feature and i need it to stay on. i searched but didnt really find anyone experiencing the samething. any ideas as to what would be causing this and if so is there a fix? thanks, any help is appreciated.