Stereo Tool

Yes, and how is that involved with "ASIO"?

I thought the problem comes from ASIO(4ALL) because when i moved the buffer offset the problem disappered (temporary).
And ... yes, i think i simply forgot to say that i use VAC for the input.

I know this is a very old post but in the meantime I found out what coused the truble, it was neither Stereo Tool nor VAC.
The problem was the buffer drift between VAC and my sound card because they run under different clocks (VAC with the CPU clock and the sound card with the PCIe clock). The solution was to add another VAC Cable and put Stereo Tool between them and use the Audio Repeater to stream the audio to the soundcard because the Audio Repeater has an automated clock correction.

Side note that in a much earlier release, I observed that the CPU load would increase if only 1 channel of ST had audio present. Attempts to duplicate that in recent versions didn't exhibit that behavior.