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I would look for a framer to look at this. most likely you have either some thermal movment, settling, or it may just need braced against small movements cauise by other forces (wind etc..). You could fix the ceiling endlessly, but until you find the issue the ceiling is just the symptom.
Cracks can occur for a multitude of reasons. I would recommend hiring a home inspector to take a look at your situation and get his professional unbiased opinion.
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