It seems to me that this roaster heats up too quickly and then slows down too much. This may be symptomatic of all popcorn poppers? As an earlier post said, it's probably true that the outside may look dark, but the inside of the bean isn't roasted enough. I may do some more surgery to gain control over the thermal override switch and manually turn it off and on to control the temperature ramp.
As of yet I'm not satisfied with the coffee I get because I can't yet get what I want on purpose. I have by accident!
I have never used a thermometer, but I've been really happy with the results I get both from the Poppery and Poppery II and even from the Kitchen Gourmet (Walgreens). I feel like with these the heat is consistent and the coffee is roasting at a good steady pace- the proof of this for me is when I dump the roasting coffee as second crack is beginning and the cooling process takes several minute. Recently I've been buying up every popper I see at thrift stores etc. and am selling them to friends who are interested, so I've had the benefit of trying all kinds of different roasters Poppery [my favorite by far], Poppery II, Popcorn Pumper, Kitchen Gourmet) http://www.thepeoplesroast.com/resources
<--- I haven't experienced the problem you mention with any of these.
One huge factor that I've noticed is the amount of coffee in the popper/roaster. I've had the experience of not putting enough coffee in so that it was spinning too fast (not at first but after a few seconds of drying out) so that the coffee was sort of cooling and roasting at the same time. A good steady movement of the coffee that is not too fast is a sort of happy medium (especially with the Poppery II) that I've found keeps the coffee roasting, even if at the very beginning it feels like the popper might be overloaded (I've had to help the coffee keep moving, but literally for only about 5-10 seconds).
My first roaster, the American Era from CVS, though I liked it at the time, did not roast a very high capacity at all. I wasted a lot of time roasting in that thing and I thought I liked my results until I got into some of these classic roasting poppers.