I've seen this problem elsewhere and it's driving me crazy on my 3G. Am going to the Apple store and asking for a replacement. It's really bad and shutting down the phone does not fix the problem.
Just got my iPhone 3G and downloaded some music. When I play it back though, rather than the song continuing without interuption, I find that my music will start and stop for no apparent reason. Then I have to hit the 'play' button to make it start again. Does anyone know why or how to make it play seamlessly? Thanks.