I have the problem on my Maxx and like all of you if I lightly tap it on my hand/desk/knee ect..will come back. Now a while back on a thread like this and I saw someone had traced it back to the vibrator not being properly lubed from the factory. He said he took the back part off and lubed the vibrator (to be honest I don't remember what he used to lubricate, it's been maybe a year since the thread) and after that he never had a problem, there were also a couple people in the thread that took his advice and they all agreed it worked. I never tried it myself though as I don't feel comfortable taking a part my phone.
Sorry I know this is very vague but hopefully it will point y'all in the right direction? lol Idk