iPhone 3GS ringer switch going back and forth on its own?

either the headphone jack/volume/power/mute flex is the problem or the motor itself. I would imagine after 2years of mute on/off that the small switch on the toggle may have been damaged or it has popped out of it's socket (it really is very small and is not glued to the outer switch). impact damage doesn't have to be great, simply dropping phones into jacket pockets has done damage before. You can open the phone and check the mute switch (quite a tricky task) if you are going to check it then i'd suggest replacing it. Then you've lengthened the flex life, and if the problem persists you can change the vibrate motor. (or you can do things the other way round..the vibrate motor is by far the easier part to replace!)