Vostro 470 Stuttering Audio

When playing video (eg YouTube or DVD or local mp4 files) or listening to audio (eg mp3) the sound stutters occasionally - usually after maybe 15 to 30 seconds of playing. Then it plays some more and stutters again. Occasionally (but rarely) I can play a file to the end without problem, but that would be the exception rather than the rule.

This is most frustrating - I bought the Vostro to replace my previous PC (lesser spec, but managed to play video and audio without problem) as I wanted to improve my video and audio editing and playback capabilities.

Any assistance would be appreciated.

Jimmy

System spec below:

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Vostro 470
Win 7 Pro 64-bit SP1
Intel i& 3770 Ivy Bridge
8 GB dual channel DDR3 RAM
Dell 0YJPT1 motherboard
nVidia GeForce GT640 1024MB Graphics card
2 x Dell U2412M 24" monitors at default 1920 x 1200 resolution
RealTek High Definition Audio - line out connected to HiFi amp
System drive: Seagate SATA III 1 TB
Data drive: Hitachi SATA II 2TB
CD/DVD RW optical drive
CD/DVD RW and BD-RE optical drive