Sounds like expansion and contraction of the wood during heating & cooling seasons.

Wood changes size (and shape) depending on the humidity it is exposed to.

It shrinks as the water content declines, then expands as it increases.

You might get a simple hygrometer and see how wet and dry your house is throughout the year.

Add a humidifier right now during this winter's season and see how much it helps. After you see that, then you might want to install a whole house humidifier on your HVAC. And use your AC in the summer. If you are in a location where you need little AC, then look at doing a dehumidifier in summer for those temperate but muggy conditions.

That sounds like truss heave. Where the trusses move seasonally or because of differences in humidity.

The idea of fastening the crown to the ceiling trusses and letting it float on the walls is the easiest fix. The only gotcha is on the two walls that are parallel to the trusses. You may have to add nailers above the sheet rock for the ceiling.

Hi, I got around adding nailers. Moulding is pretty light.
I put LocTight glue on the moulding held it in place and used a air gun and shot in the nails at opposing 45degree angles. Worked great.
Good Luck Woodbutcher

Several already replied about the trusses. Search Google for "truss uplift".