sound card output on oscilloscope skewed.

Hi Everyone.
Im driving an oscilloscope display, in XY mode using a high frequency audio signal direct from a soundcard.
(left=x, right=y)

Ive done this successfully before, but Ive since swapped PCs.
The resulting image is now skewed about 30 degrees.
(Its not a straight rotation, more like a skewed scale).

The strange thing is I am using the same external USB sound card that produces perfect results on my previous computer. (I also get the same skewed results with the new PC's internal sound card as well).

Both scope + PC are plugged into the same powerbar, and I tried feeding the signal through
a headphone amp with the 'ground lift' button enabled,
and well as wiring a clip between scope + PC chasis with no change.

Any clues what may be causing this distortion?

Thanks very much,
Rob.

For anyone interested: My software defaulted to outputting Front-Left, Front-Right, Back-Left, Back-Right by default, which mixed some of the separate channels
before outputting through the L, R outputs. Changing it to stereo got clean separated signals again..