BT.
"my "headphone test" is showing no glitches"

That is very odd. So no funny stuff in headphones connected to Tascam?
Sure, sure?


Best move now would be to test it with i5 machine. If there are no issues, you will eliminate Tascam unit (cable and power supply) out of equation in finding the boogie man.
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Something that might relate to you (or might not) . Last time I fought similar issues was because of Nvidea chipset. I have Thinkpad (Lenovo) and they, as Dell are pretty good in maintaining drivers, BUT that specific driver was not up to date on their site. Once I updated that specific driver, things became much more stable. So check if you have Nvidea or other graphics card.