Screen on MAC Pro keeps flashing

Swap for a different cable and for different display, and — if those swaps fail — you're headed for repair or replacement of the Mac Pro itself.

With the use of macOS 10.14 here, I'd guess this is a 2012 or earlier "cheese-grater" Mac with Metal graphics, so cleaning and re-seating memory and boards is an option for some folks, then maybe then swapping graphics, if you want to spend time and money.

great advice provided By MrHoffman above.

Mac Pro with Xeon processor has Error Correcting Code RAM memory, so don't spend more than a moment fiddling with the RAM. Absent obvious issues such as kernel panics, RAM problem on these models simply do not fester undetected.