Rather odd that this affects a card with an EMI shield (even though they are not the final word in stopping EMI), but does not affect onboard audio, which more often than not is worse for picking up EMI than any sound card.
I remember there was a member some time ago who had the same problem with his STX; interference on microphone but not the audio output. He never did manage to solve the problem; I don't know what he ended up doing. He had the issue with onboard as well though, from what I remember.
OP, can you not just use the microphone out from the motherboard and continue to use the STX for audio for headphones or headphone part of your headset?