One of your best, in my opinion. It's a shame the quality ain't that great because the way the spread of colors shows up on this flame is really extraordinary. Definitely worth a re-render and some postwork in the future.
Yeah, the squares tend to lose quality towards the middle. Reducing the size helps, but in the end there would have to be some form of blurring. I tried to do some selective gaussian blur in GIMP and it worked alright, but didn't have the same look.
the grain comes from the nature of plugins and can't get rid-of it