Clip+ stutters with speakers

I’ve just bought a Sandisk Sansa Clip+ (8Gb). It works fine with headphones, both the supplied ones and others. But when I plug  in a speaker or an amp instead of phones, it stutters - the sound drops out for brief moments. Any idea why?

The plug (from the speakers) may not be seated completely in the jack. They are especially tight when new.

That seems to have solved it - thanks!