track stops short then another one starts

[longshanks]

My Zen which does not get a great deal of use has alway's had this niggling problem.
Some tracks whilst half way through playing end and another track will start, it's as if the Zen is trying to play a sampled composition, this is intermittent. I have tried all the re-loading of firmware ect but that has'nt made any difference.
any idea's please.

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