ya, like i said, im not an expert by any means about selonoids...so its just my objective opinion...
grounds have a tendency to loosen up because lots of people use self-tapping screws which DO loosen up over time...
even I, after years of installing was using self tapping screws until i became MECP certified, now i realize that they arent the best solution. anyhow, try to check the ground, it may just be a bit loose and there is all your problems...also, did you use a star washer when you grounded?? they help to 'cut' into the metal a little to make sure you get a solid ground.
http://www.midwestmil.com/lockwashers.jpg
let us know how it works out..