It sounds to me like a problematic wheel, since I am about your size, and have 3,000+ miles on a Giant Sedona (which has wheels of similar quality). I have had them trued twice, once because of a broken spoke.
I would ask your LBS if they use a tension meter when they true the wheel, as I understand that could help it to hold a true better.
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Slow Ride Cyclists of NEPA
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