When I start up QJackctl, everything runs fine for a while (albeit with more xruns than seem reasonable), and then after a couple minutes of recording through my USB soundcard, the input sound turns into electric garble. (Not just a little distortion, more like complete ringmod garbage—maybe a cool effect, if only I could control it!) If I wait for several minutes, it returns to normal, and if I restart JACK, it returns to normal. And then it turns to garbage again.
I'm running 18.04 on a Lenovo G580. (It happened on previous versions of Ubuntu, too, but never on my former laptop.) The problem doesn't occur when I stick to internally generated sounds (like playing an audio file or midi sounds), only when I go through the USB port, which I've tried with two different sound cards (an Alesis iO2 express and a generic card), and JACK. I recently added more RAM, and that didn't help. I tried killing PulseAudio and that didn't help. Reinstalled Ubuntu and that didn't help. At first, I thought it might be a driver issue, but I don't see any other G580 users complaining.
It's as if some program that starts and stops every few minutes (maybe even runs on a schedule?) is interfering with JACK, but I haven't been able to come up with any candidates from Task Manager or the QJackctl Messages box. Any ideas what it could be or where else I could look?
Thanks. This has gone on for years now. . .