LG UltraFine 5K with MacBook Pro 2017 issues
It sounds weird but anothet thing to try is if you have the Macbook touching the LG5K display in any way it causes some sort of earthing issue.  As you know the Macbook aluminium case is a conductor and there are minute amounts of current generated on the shell by the Macbook Pro itself.  If these currents find their way to the LG5K display it causes erratic behaviour (display turns off for a second or two) or wavy lines.

Also pls be aware not all Type-C's are the same - a Type C1 (TH3 mode) passes a 5K display signal whereas a Type C2 (USB3.1) does not.