I was having similar problems with my Gene, longish roasting times. I asked a neighbor, who is also an electrical engineer/instrument control specialist, whether he had noticed any voltage line drops at his home during the past months. Long story short, he "lent" me an old Powerstat variac to use to see if that would fix my problems.
Been using it for more then a year. If he knocks on the door today and asks for it back, I will be on Ebay 5 minutes after that buying another!
Prior to the variac, I had voltages drop to a low of 115* with the Gene heater on, to low (IMO) to roast without concern I was baking the beans. I use the variac and Gene configured like this:
Gene ----> Kill-a-watt------>Variac----->outlet
and with the heater "on" (as opposed to cycling), dial the variac until the KAW is reading between 120v - 121v. I read that today's electronics can handle swing of +/- 5% voltage. That makes the range of acceptable voltage 114v-126v. When the Gene reaches "set temp", and the heater cycles off during this brief period, the KAW sometimes reads 125-128, but this is only for 5-15 seconds at a time. I guess I could adjust to keep it within range each time the heater "cycles off", but I would look like a mad scientist rotating the variac dial that often during a 10-14 minute roast.
Made a huge positive difference on my roasting.