i had this problem with 2 different 126´s yet and the new one that
worked until 3 days ago start to have this problem as well.
i think it was not the power chord but when you removed the module from
the chassis to verify the cable, you accidentaly "repaired" it.
there was a thread about this problem before, the member described it as
"pushing the ic´s gently in their place" which helped me at the first
time as well.

in my case the mixed, up and down outputs are very silent, almost mute
and the direct out (audio out) works as its supposed to.
but no shifting at the wet outs at all.