My miners would always drop a GPU over time. Usually GPU2 but not always. I have seen this with Claymore and optiminer so it is an Ubuntu and or AMD driver issue and not the miner. Below are some tips I have learned and a step by step for a rock solid optiminer 1.4.0 using RX480 8GB cards getting ~300 S/s / card (2,100 S/s for rig with 1,100Watt at the wall).
Hers is what I know:
Ubuntu 16.04 server is very difficult to install and get working with a 7 GPU rig (or even 6 for that matter). I have got it to work but there are so many issues that need to be fixed its just not worth the time trying. Ubuntu 16.04 Desktop installs with relative ease and has just a few minor issues.
So here is a step by step to setup a stable optiminer 1.4.0 rig:
1) install Ubuntu Desktop 16.04.1 with no GPU's installed
2) install open ssh, Byobu, vim (unless you like nano), and lm-sensors (you can do all from ssh from this point)
3) perform and apt update upgrade and reboot.
4) install am-pro 16.50 drivers, add yourself to video group, and shutdown.
5) install all GPU's and reboot (check that lm-sensors shows all your GPU's. If some are missing a reboot will usually fix this. On my rig a boot with the ethernet port disabled in the BIOS will get my 7th GPU to show. Then reboot and enable the ethernet and the 7th GPU will stay (probably particular to my MBO but need to free up something off the PCI bus)
6) Once you are at this point enable Byobu and go headless via ssh
7) Set the default GRUB behavior to boot into a console AKA, do not load the desktop / GUI.
8) Load optiminer and your run scripts.
9) via ssh and Byobu start your miner and fine tune fans, verify S/s, temps, ect ect (system check)
10) reboot via ssh and Byobu and start your miner.
11) immediately disconnect via F6 and leave session running.
That is it. Leave it run like this and monitor you S/s via your pool statistics NOT ssh. If you do connect via ssh to check on things or perform maintenance, reboot, start your miner and then disconnect again as in step 11. if you see a GPU go down, try step 11 again until things become stable (then leave it alone). I was restarting almost every day but now my miners just run and run and never drop a GPU. However, if I connect to a rig via ssh I can drop a GPU fairly quickly just as before, even if I disconnect quickly.
A couple pointers:
do not use sudo reboot. Instead use sudo shutdown -r now, I had issues with reboot hanging, especially when a GPU dropped and I was out of town. If you do need to remotely ssh into a rig shut the miner down as soon as you connect. A soft reboot can fail with a dropped GPU so its better to shut down the miner before than can happen. Always reboot, start miner, and immediately disconnect when you are done. If you want to monitor things a little closer then setup your rigs to email you performance statistics. The trick is don't use a GPU for a display or ssh connection and things will just keep running.
Thats what has worked for me and I have not had any more issues. I would expect Claymore to be stable doing things this way as well.
I am having an issue with optiminer 1.5.0 that drops my performance to ~1 S/s card no matter what I try so for now I will stay with 1.4.0 and troubleshoot 1.5.0 another time.
Hope this helps someone.