Check your UV layout of the low poly geometry, this has to be 100% clean, otherwise you may get undesirable results. Also I take it that there are no massive differences between the high detail geometry and the low poly. Things that would cause streching etc... One last thought would be to decrease your bumpmap depth level.
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Chris (formerly R@nSiD)
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Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)