The "feel" of my guitar (and every GJ guitar I've owned) changes from day to day, even from hour to hour. It seems to be based on the relative humidity and maybe the temperature. I can tell as soon as I check the tuning and give it one strum. Seems like the action may change a bit, but it's almost unmeasurable -- a 5th of a millimeter(!). Sometimes the strings are so loose I can't play smoothly (especially upstrokes). Since the action barely changes, I was wondering if the top becomes flatter when the humidiy drops, and the neck comes up, so that even though the action remains almost the same, the "break angle" over the bridge decreases. If that's what's happening, the strings might have a looser feel because they slide over the bridge when they're played, making them too "sloppy." No other guitar seems to have this issue for me.
Also wondering if skme GJ guitars seem to be more "stable" than others? If this theory is correct, maybe those with a greater break angle over the bridge.