Hi there,
i'd suggest you get ASIO instead of WASAPI. For me and my external DAC it worked just perfect. I never had a sound this stable before.
"ASIO4ALL V2" - Universal ASIO Driver For WDM Audio.
After installing, there's a manual on your desktop which describes every function in detail and how to set it up. Normally, a smaller buffer size stands for low latency and an increase in jitters, clicks, ...
With Asio4All you can decrease the buffer size to a minimum of 64 samples or up it to 2048! Maybe this will do the trick.
Hope it helps!