Yeah, every individual oven seems to have it's own quirks, but it seems to me that the two main factors are the area of the vent and how many cubic inches you have created as a roasting area (based on the size of the riser). I usually use a 2" riser made out of aluminum with about 1/2 square inch vent area. I tried a 3" riser once but it slowed the roast down too much. The vent dynamics can also create hot spots inside the roaster. I had one stir crazy that had a big crack in it that was allowing air to blow out under the riser, and that created a hot spot that resulted in uneven roasts. When I noticed that, and taped it over with aluminum tape, it went back to normal.
If you frequently blow the oven out with compressed air to remove chaff and dust, it will last a lot longer and work better, too.
Galloping Gourmets always seem to be more durable to me. The digital Sunpentown (SO-2001) and the Aroma were the two least durable I have used. I always have a couple of backups laying around, so when an oven starts wussing out after about 800-1000 roasts I usually just shelve it and grab another. My main Galloping Gourmet that is in use now probably has 1500 roasts under it's belt, and is still blowing hot.
BTW- I never disconnect the SC heater. It helps dry the beans quicker, and since it only goes up to about 350, it doesn't significantly impact the main developmental parts of the ramp. With a 20oz or 24oz batch, the water would probably get driven off too slowly without that extra power.
