I had a similar problem, where Audacity had been recording flawlessly before. The sudden stops began after I had checked Software Playthrough of input in Preferences, under recording. I unchecked it and was again able to record without interruptions. (All other Recordings preferences remained unchecked.)
I was recording audio from a vinyl record through a USB analog-digital converter. Sound levels were set by the excellent Pavucontrol (PulseAudio Volume Control) tool.
Apparently putting the input through the headphones in order to monitor sound quality while recording is asking too much of a run-of-the-mill laptop system like mine. A minor inconvenience, but I can live with it.
Sudden unwanted recording stops were also discussed here:
https://forum.audacityteam.org/t/audacity-stops-recording-after-about-six-seconds-solved/54163/1
Audacity 2.2.1 - Linux Mint 19.1 Tessa 64-bit - 7.7Gib - Intel i3-2350M @ 2.3GHz x4