[Electriclight]

Seems like you have covered all the bases. Here's a long shot in the dark...What sample rate are you using to record? Is there a chance there is a mismatch.

Also, although it seems intuitively right and there is such advice out there, I'm not sure you actually want your Firestudio as your primary sound device in Windows. At the very least, turn off all sound schemes in Windows.

Has anybody considered you may have a lemon Firestudio? Has it ever worked?