Arduino Yun keeps rebooting after some time

Hi everyone.
I am currently struggling to troubleshoot my project wich is based on an Arduino Yun.
I am running a rather simple program on the yun, reading out an Analog Sensor and sending it to Node.Js via Serial1. (even put in a delay of 100ms to limit the values sent)
Node.js then forwards these values to a webclient via to a webclient on the devices connected to the wifi.

My problem is, that after some time (after all I tried it's still between 15-30 min) either just the wifi or the whole yun reboots.and of course the clients get disconnected which is a big problem. Is there any way to find out what caused that reboot? Are there any crash logs on the yun?
Any Idea what could cause that problem?

I already took out that wifi-reset line in rc.local which was suggested somewhere. The wifi is even reset to the default access point settings. Still the same problem. So I am hoping to find out more with a crash log or your help.

I first had only 50ms as a delay in the Arduino Code and that wifi-reset in and think the crashes were happening more often. Now the time it runs is longer but still not stable.

If I can't get rid of this I have to switch to a Raspberry+Wifi+ArduinoUno Setup which would be rather messy for that "simple" task. Would want to avoid that.

Re-flash clean firmware:

Find out how long Yun is running:

root@Arduino:/# uptime
 21:07:00 up 5 days,  1:49,  load average: 0.03, 0.06, 0.05

Change power supply of Yun:

USB cable, power adapter...