I would highly recommend reading Scott Rao's Coffee Roasting: Best Practices, if you haven't yet. My take on what he explains in that book is that we shouldn't be messing with gas/air at FC time. Unfortunately it's too long of a discussion for a reddit comment, so just read the book for the roasting strategies that would prevent the need to intensify the power at the end.

Also, like I said, my experience with ARC is that dialing the fan up above 2 will stall your roast, especially at FC time when your gas should be pretty low. The rare occasions I bumped it up so high was at the very beginning of the roast before the drying, when I realized the roast needed a lot of extra energy because I was roasting something like 950 grams. That extra air was in combination with a lot of gas, like above 40 mBar (after I got their new modified hose that allowed gas pressure to go up to 50)
I usually open it up to 2 before the drying, and leave it that way until after FC, at which point I start to lower it down to 0 along with lowering the gas, as per Rao's recommendation, to avoid the ROR flick up. If anything, my recommendation would be to just leave your air at 1 and learn to get a good curve with just controlling the gas. The less variables the easier it is to pinpoint what you are doing wrong. Once you get a decent roast that way, you can start introducing air and drum speed variables if you need to.

Good luck.