@wheetabix - I know your pain! I made the journey to the local Apple Store 4 times.
This last time they were leaning towards my MBP as the source of the root cause. I convinced them that replacing the TBD would be the easiest way to prove that. As opposed to digging into the HW/video card on the MBP or flattening and rebuilding OSX - which with the MBP being maybe 45 days old was NOT a pleasant option.
With no flickering so far on the new TBD I'm of a mind that the TBD was the source of the root cause. But, more time is needed to really prove that this TBD is free of the issue.
Should it start to flicker trust me I'll post such bad news here.
Since Apple already replaced the logic board on the TBD and that fixed nothing replacing the LCD panel in the TBD was the next step. I'm sure 90% or more of the cost of a TBD is in the LCD panel.
Giving me a new TBD for Apple was a bit of a wash and helped to repair my relationship with Apple. I'm a long time Apple customer. We have a ton of their equipment in our business. This last time when I visited the Apple store I was both polite and emphatic that this issue needs to resolved. I was calm, I was polite but I was also very firm that I was unhappy.
The staff did a great job of listening, empathizing and ultimately replacing my TBD.
You want to know why this was so frustrating? Because typically Apple's products for me have been rock solid. Over the years we've had a few hard drives go away. But that happens across all platforms. Other than that it's the solid reliability of Apple that keeps me coming back to them.
So when this happened it really cranked up my frustration level!
I'm happy today. And hopeful that I'm on the backside of this experience. And I'm still a dedicated Apple FanBoi!