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I've got a MacBook Pro (15-inch, 2017 model, from work) plugged into a large 4K external monitor via a USB-C to DisplayPort cable.

It's worked nicely since I started using this setup a few weeks ago, rendering 60fps at 4K with no issues. But something strange has started happening today: the entire screen flickers green whenever CPU usage is high. It flickers briefly when I scroll complex web pages, for example.

I suspected the flicker was related to CPU usage, and used Activity Monitor to confirm this. When I open the App Store, for example, I can see it consumes a ton of CPU on launch, and the flickering gets much worse at exactly the same time. If I'm doing non-intensive work, like typing in this box right now, there is no flicker at all.

Does this indicate a possible electrical fault in the MacBook Pro? I considered whether it might be a dodgy DisplayPort cable, but I guess the signal coming through this cable shouldn't be affected by CPU usage, unless there's a fault in the laptop itself. Could it be that the graphics card is becoming underpowered while the CPU usage is high?


UPDATE: I switched the DisplayPort to USB-C cable for a new one, and it solved the flickering. But this still concerns me: why should the DisplayPort cable be affected by CPU usage at all? I'm concerned this indicates an electrical fault, which may have cause the cable to become faulty (so perhaps it will damage this new cable too over time).

The flickering with the original cable happens when the laptop is powered by either mains or battery.

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I saw a similar issue with a non-Mac laptop and external screen caused by an earth loop. Easiest way to check is to try and trigger the issue with MacBook on battery power only (no charger connected). If you can't trigger it then try a standard figure-8 power lead instead of the 3pin adapter in the charger (the earth pin is connected to the metal pin on the charger). If it won't trigger, then that is the issue, will be due to poor ground/earth isolation in the Mac or the TV to the connector.

The only solution then is to break the loop, simplest/safest way would be to continue to use the figure-8 power lead. Maybe apple sell an un-grounded plug adapter (US and EU versions don't have an Earth).

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  • It is the same even when not plugged in, when running from the battery. – callum Jan 13 at 10:06

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