Are you running on USB1.1? The RPI USB 2.0 implementation does not work well with full-duplex.
To switch to USB1.1, add dwc_otg.speed=1 to /boot/cmdline.txt.
However, at least the UCA202 I have tested, does not work with USB1.1.
The only sound card that works on USB1.1 and has stereo line-in I found so far is LogiLink UA0099http://www.logilink.eu/showproduct/UA0099.htm?seticlanguage=en