I was experiencing this problem too.
It appears to happen when you hit a bump that's bad enough to shake the iPod just sligktly loose from the cradle.

I took a velcro strap to it (to hold the iPod more securely in the cradle).

Seems better now...

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2006 4.4i
Jet Black/Truffle Brown
Cold/Premium/Rear Climate/Sport
Adj Ride Ht./Navi/Premium Sound
IcelinkPlus/iPod