Audio Output Hops From Headphone To Speaker??

[emmet]

Hi,
Using a Powerbook G4 12" OS 10.4.10.
I was trying to play something back with quicktime and the sound was hopping and after a long time trying to work out all that rubbish about codecs I realised that in my sys prefs, sound output was hopping between Headphone and Speakers every 3 seconds, this is making it impossible to play stuff back.

Theres never a dull moment with this heap of Apple ****

Thanks in advance!