After rereading your post, it sounds more like a hardware issue with the device. After you updated the Firmware on the Phaser, are you still getting lines on the devices LCD screen ?
When this was happening in the past you rebooted the print, did the lines disappear or were they still presant. Are you getting any type of error codes ?
In the past, I had some episodic problems with my Phaser, which sometimes didn't print the document. When this problem arises, I did never found why but sometimes, the phaser completely hangs (showing some strange vertical lines on the LCD screen) and the only solution was to turn it off and on again, then restart the desired printing. Sometimes, restarting the printer doesn't work so I have to delete the printer in the System Preferences and install the driver again. Sometimes, it solves (temporarily) the problem, sometimes not...
But today, the situation is getting worse : I cannot print anymore, with any of my computers. I tried all the day long different protocols, different installations, reboot computers / printer, nothing is printed anymore. I don't get any error message, neither on the computer nor on the printer.
My printer is connected by Ethernet and gets its (static) IP address instantly - no problem here. I can access its web interface and it's fast.
I have computers running MacOs Sierra and MacOS High Sierra.
I configured the following protocols on Phaser : AppleTalk, TCP/IPv4, HTTP, SNMP, LPR/LPD, SLP.
I use the latest version of the driver (downloaded today) and the Phaser 6700 has the latest firmware : 081.140.103.22600.
There is toners and consumables are ok.
What can I try ? What can I check ? I'm stuck...
Thanks for your help,