What can be done to diagnose node server suddenly failing to respond to requests

I am using pm2 to run node in production, and the server will suddenly stop responding to requests after a couple of hours. Pm2 says the process is still online and everything is good. The logs don't say anything relevant. There seems to be free memory:

             total       used       free     shared    buffers     cached
Mem:       3840472     653388    3187084          0      65356     332088
-/+ buffers/cache:     255944    3584528
Swap:            0          0          0

Is there any ideas as to of what I could do to diagnose the issue?

PM2 Config:

[{
  "name"             : "app",
  "cwd"              : "/var/www/app",
  "script"           : "app.js",
  "error_file"       : "/var/log/app/platform.stderr.log",
  "out_file"         : "/var/log/app/platform.stdout.log",
  "run_as_user"      : "ubuntu",
  "instances"        : "max",
  "cron_restart"     : "1 0 * * *",
  "watch"            : "false",
  "ignoreWatch"      : ["[\\/\\\\]\\./", "node_modules"],
  "merge_logs"       : "true",
  "exec_interpreter" : "node",
  "exec_mode"        : "fork",
  "env": {
    "NODE_ENV": "production",
    "PORT": "80"
  }
}]