I agree with mechBgon. It's more than likely a spoke problem. I think it is also caused by the terrain you are riding. For wheels to remain true the spoke tension has to be fairly high and uniform around the entire wheel. When it is not, the spokes have a tendency to loosen under severe road vibration caused by the uneven conditions you described.
If you are truing your own wheels, make them tighter. If an LBS is doing it, I don't know what to tell you.
By the way: WELCOME! to the Forums. You've chosen an interesting name.
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ljbike