My LG 5k UltraFine monitor is blinking on and off (about every 4 seconds) when connected to my Mac Pro 2019 with W5500X video card and latest operating system. Have replaced the thunderbolt 3 cable, no fix. What can I do? Is the monitor broken?

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".

The cables that clearly state they fit the high-end hi-color situation are said to support HBR3:

https://www.apple.com/shop/product/MQ4H2AM/A/thunderbolt-3-usb%E2%80%91c-cable-08-m

Thunderbolt 3 Pro Cable (2 m) - Apple

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Just a guess. Make sure you are NOT using the three USB-C ports on the display.

EXACTLY what ThunderBolt cable, where did it come from, and how long is it? {up to 1/2 Meter, longer than 1/2 meter}