Hello,  My Roon Nucleus+ is hardwired with cat 8 from my Verizon Modem.  I am then hardwired to two Roon endpoints.
I use a separate linear power supply for the Nucleus set to output 19v.
There is an internal hard drive installed and NAS connected.
Everything sounds great.

But my Nucleus+ is crashing now two or three times a day (becomes non responsive and my remotes can not find it.

I have tried rebooting the remotes, the endpoints and the router with no effect.

I must power off and back on the Nucleus for it to be ready and seen again.

Any help or thoughts appreciated!
I've had a similar problem, though not nearly as frequently as you described.  My iPad is unable to connect and I end up rebooting my Nucleus.  Looking at the Roon blog and emailing them indicates the issues is likely to do with DNS.  Some dynamic ip allocation systems don't seem to play well with the Nucleus.  One approach is to assign a static IP.  I haven't done this for other reasons and my problem isn't as frequent as yours.  Take a look at the Roon blog and/or send them a question.  It can be resolved.

Hope that helps,
Thank you both! I have similar support request in at Roon just have not heard back yet....
This didn't occur too often for me until the recent 1.8 upgrade. Now I'm seeing it much more often. I'm thinking that the 1.8 release was premature.
You might also look at the M.2 SSD. If this is starting to fail, this will cause the Nucleus to crash. If you connect a monitor to the Nucleus when you boot it, the BIOS may give you some indication of a hardware problem. 

I had a problem with my Roon ROCK NUC which I initially built with a fairly cheap M.2 SSD. It was crashing once or twice a day. Sure enough, the BIOS was reporting problems.

When I replaced it with an Intel 660P M.2 drive, my NUC has been flawless. I have only had to reboot for software updates since then (last six months or so). 
Just because I am super suspicious of little devices, put a fan on it and see if this magically improves.
Might be a modem issue. If controllable by the user (you), check for software updates for the Verizon modem (and update). Otherwise, ask Verizon Tech Support to flash the modem.
Recently my Ayre DAC is no longer being recognized on ROON. I have contacted ROON and truth be told the person with whom I've been in email contact,
has been less than helpful/knowledgeable. I spoke with Ayre and they thought that a re set of the DAC  would fix the problem. It didn't. Thankfully I can speak with a life person at Ayre. FYI, ROON customer service personnel don't work on weekends. Good luck to us both.
Like my tech buddy says “ if you got a computer, you got problems”. Probably modem/nucleus interface issues. 
IME, Roon’s customer support is lacking. After e-mailing them with a question and an explanation that my specific query wasn’t addressed on their forum, I waited a long time before receiving an e-mail suggesting I look for an answer to my question from the amateurs on the forum.
Reading this thread, it’s always a reminder of how easy it is to pop a CD in a tray and hit “play”
Roon support should turn on diag to isolate the issue.
I had a similar issue and diag showed the SSD was up but logging errors, a new SSD corrected the issue.

Roon support was slow to respond but accurate. Once diagnosed I was sent a shipping label, 7 days after shipment it was returned to me with a fully functional SSD.

It was under full warranty, beyond time my cost was zero.

^THIS^  Roon diagnostics is the direct route to an answer.  Roon 1.8 would not cause only your Nucleus to crash.  Mine was an M2 drive problem as well.  I could have guessed and fiddled instead, but Roon support found the problem.
Thanks again to all...I am waiting for the next crash to alert ROON so they can investigate.

Of course it has been flawless since I contacted them!