iPod app shows run time incorrectly iPhone

Im a big audiobook fan and sometimes get them on CD when theres no download available. I ripp them myself and merge all the tracks so I get one nice clean file for the entire book. As you would expect, these files can be quite large.
They work fine in iTunes (or any other desktop audio player), but when I transfer them to my iPhone 4 (latest version of iOS) the iPod app will at first show the run time correctly, but then a second or so after playback begins, will then re-calculate it to a massively incorrect figure.
For example, I imported a new book today with a run time of 13:49:04. When I hit play it then shows the correct time briefly, but then quickly switches to a run time of 73:33:51. Then time slider is for this for the full imaginary 73 hour run time, so only the first fifth of the time slider actually works.
Most odd, and Im assuming its just an iOS bug with no fix...?
Books bought online (from Audible for example) work fine with long run times...
Shuttle SB61G2