I have a home-modified air popper that I use to roast coffee and I don't get first crack if I put too much coffee in, turns out that the limit of my machine is about a quarter pound at a time, otherwise, I don't get enough agitation for a fast, even roast. Maybe try reducing the amount of coffee you're roasting?
Another thing that can cause an issue for me is too long of an extension cord. I'm right about at what the limit of my machine can handle and the slight drop in current caused by the long cord is enough to lower the air temperature about 20 degrees.