The 'last packet sent time' is bugged right now (it will update on a browser refresh when your Pi is offline) so I wouldn't rely on that for troubleshooting. We should have a fix out for it next week (currently sounds like Monday)
That same update next week should also have improvements for memory consumption and disk / performance issues do to runaway log files.
I'm not sure if any of those performance issues is the cause of this behavior you've described though. I wouldn't think that even if our software was pegging the entire machine to 100% that this would block SSH. That behavior with the mem/CPU/etc widgets going to zero makes me think that whatever is blocking SSH is probably also blocking the Cayenne agent from communicating with our server.
A shot in the dark, but could you give your output from
uname -a? I know the Cayenne agent has issues with the (still expirimental) 4.9 Linux Kernel -- that output would allow us to see if you've gotten updated to that somehow.