I believe your "shipped in the box" cable (notorious in general for being a "lowest bidder" cable) may be inadequate for Full "5K" at 60 Hz hi-color. the correct ThunderBolt-3 cable has a ThunderBolt at each end. [The other possibility is that you were using the inadequate USB-C cable supplied,
I believe the ThunderBolt cable you bought may not be adequate for this high-end situation.
The simple experiment would be to reduce the resolution or colors, and see if the blinking abates. (caution, not SCALED resolution, ACTUAL resolution say, to "4K" rather than "5K" = "best for display".