Unstable monitor outputs

I have two monitors connected to my Late 2013 Mac Pro. Generally all is fine.

For some time I have been having intermittent unstable monitor output with this Mac. Sometimes I get I judder on my main screen with some jagged sections of the image moving across each other and at other times the images flicker between my two monitors with neither stable (often after the Mac wakes from sleep). Often pulling out the relevant monitor cable, or one of them in the case of the flicker between monitors, and then re-inserting, fixes the problem and all is well. I also thought a PRAM reset made things more stable for a bit. But nothing seems to really see this issue off. I know that Late 2013 Mac Pros did have an established issue with stability of the graphics card which may be what is going on here but it's so intermittent.

Any advice?

I am using very high quality monitor cables by the way. One is connected via DisplayPort cable (with a Thunderbolt 2 adapter) and the other is HDMI.

Thank you.

Mac Pro (Late 2013) 3.7 GHz Quad-Core Intel Xeon E5

64GB 1866 MHz DDR3

Graphics: AMD FirePro D300 2 GB

macOS 11.3