It sounds like your hub caps may be rotating on the rim. An out of balance condition, loose fitting or incompatible hub caps can cause/allow this to occur. We had this on our Avion and actually cut a valve stem during a Fall 2009 trip.
Several remedies we've used include wheel hub balancing, a wrap of duct tape around the rim where the hub cap tangs make contact, or a piece of 1/2" diameter vinyl tubing over the valve stem.
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