The problem does appear to be truss lift and while for TomH's California region the problem doesn't normally show up in summer which is the hotter less humid season...for the rest of the US the summer is not only the hotter but also the most humid time of the season and why the lift occurs.
Wood changes shape with moisture content and the greater the mositure the gretaer the lift.
Sounds like a typical truss lift moisture change problem to me depending on the climate.
The correct fix to truss lift is to either install mrtal truss anchors to the side walls or to install the drywall so that it is not fastened to the underside of the trusses within about 2 feet of any wall-ceiling joint.
This allows the truss to deflect with seasonal humidity changes and for the drywall to stay in place at critical areas despite the deflection.
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