[LAU] How to recover from a JACK freeze?

Hi, been inactive for a while. I just up/cross-graded to Ubuntu Studio 16.04 -- very much enjoying it so far! I'm getting better USB audio performance than I could ever manage under plain Ubuntu (provided WiFi is off, but I can live with that for shows). One problem that the upgrade didn't solve is that sometimes jackd gets stuck and it can't be killed. Initial symptom: No audio, in or out. Secondary symptom: In qjackctl, the CPU usage number is frozen -- never changes. At that point: $ ps x | grep jackd 3140 ? SLsl 0:16 /usr/bin/jackd -dalsa -r44100 -p1024 -n2 -D -Chw:Set,0 -Phw:Set 4458 pts/1 S+ 0:00 grep --color=auto jackd $ kill -9 3140 $ ps x | grep jackd 3140 ? SLsl 0:16 /usr/bin/jackd -dalsa -r44100 -p1024 -n2 -D -Chw:Set,0 -Phw:Set 4460 pts/1 S+ 0:00 grep --color=auto jackd "kill -9" has no effect. "sudo kill -9" has no effect. If I reboot, it takes several minutes for the system to shut down -- i.e., jackd is even preventing a clean shutdown. So, the question: How do I regain control when this happens? (Ideally, without having to close everything.) Thanks, hjh