Ethernet W5100 randomly unresponsive as web server

I have a program that uses an ultrasonic sensor to measure the water height at my dock on Lake St. Clair and post it on a simple web page. I used the example programming to do the server parts and it works fine until I get no response after a random time; a day or two. I don't understand a lot of the programming but it works until it doesn't and then I have to go out and reset it. I added a 12 hour reset circuit to see if I could prevent the problem, but it still locks up. The unit is powered by a 6 or 12 volt wall wort and has worked thru winter and summer. Am I missing something?

Dock_Sensor_with_12_hour_reset_6_2_2019.ino (8.51 KB)

mososki:
The unit is powered by a 6 or 12 volt wall wort and has worked thru winter and summer.

What does that mean.
6volt (regulated) on V-in is borderline low, but is too low for the DC socket of the (assuming) Uno.
12volt on the DC socket or V-in is definately too high with a W5100 shield on top.
The Uno's 5volt regulator will randomly overheat and reset from the extra ~160mA current draw of the ethernet shield.

mososki:
Am I missing something?

We... are missing most of it, including the attached code which we can't see or won't open.
Read the "How to post" sticky.
Leo..