Just to follow up, I haven't had the first issue (whole screen dims) since doing the Factory Reset. I haven't had the second issue (checkerboard dimming) since I toggled Game Mode (even after turning it on again). The third issue seems like it was fixed by firmware update 1301.5 and was referred to in other posts as inverse ghosting.
Edit: I've unmarked this as a solution since other people are still having problems with the screen dimming (just hasn't happened for me again).
I updated to the latest firmware and are having random dimming issues as well. I go in and flip the Picture from Filmmaker to whatever and the dimming "wakes up" for a random amount of time. Very annoying. I'd roll back to the previous firmware if I could (even the inverse ghosting fixes are very good). Oy.
I factory reset with newest firmware and it fixed nothing. Still getting the random full screen dimming in game mode (does not happen when in my default non gaming settings movie, or filmmaker). All eco settings are off. Funny enough it seems to be more aggressive dimming on the Nintendo Switch then the Xbox One though they both do it. I really hope Samsung notices this game mode issue and fixes it. Main reason I got the Q90T was for its gaming features, but the dimming is super distracting.
Yeah, it only seems to get wierd when coming out of game mode and switching to HD Signal (my DTV Box). Swapping between "Movie" and " Filmmaker" seems to the TV. For the record, it's been nearly 12 hours since it last did this. I've come out Game Mode at least 6 times now with no issue. So who knows.. I'll let you know if I see it anymore (fingers crossed).
Tried another factory reset today. This time I switched over everything to a different hdmi port and left all settings at default while playing. Sadly Gamemode will still randomly dimmed the entire screen for a few minutes. I have tried everything I can think of to get around it. It's not the consoles, this doesn't happen when I hook them up to any other TV in the house except this one...the newest most expensive TV I bought specifically for gaming. This it seriously lame.
Yup, started happening to me again, too. So factory reset did not fix it. Same thing. After being in game mode for awhile, the screen will suddenly dim until I do something like switch Game Mode off and on or the Local Dimming settings.
Edit: Actually, before the factory reset I was getting this issue while watching Netflix on my Xbox, and playing Last of Us 2 on PS4. After the reset and firmware update I only had it happen while playing Last of Us 2 on PS4. Another person I know with the same TV has also only noticed it while playing Last of Us 2 on PS4. So I can't tell if the issue was fixed (only an issue with the game or PS4), or partially fixed.