Mine does the same thing and started to do it after install of xm radio. It means your GPS signal is weak possibly from electronic interference. No cure other then removing the electronic devices. Since my Nav works as well as before I do not worry about it, although it did bother me for the first couple of months. 
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2004 Lux with 6 inch Fabtech with tie rods, 38 inch MT's, 4:88 gears, and some minor engine tweaks.
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