If your oven is that wonky you probably should not try to bake polymer clay in it. Most polymer clays bake at around 300-350 degrees F, so if it was burning in just fifteen minutes your oven was probably about 400 degrees F or more. If you do opt for the "craft oven" option, which I would recommend, make a little "tent" for your clay out of aluminum foil; this will prevent scorching which would be especially noticeable on white pieces. Just be careful not to let any pieces touch the foil.
Lol, Dolly Kim, I never pre-heat my craft oven; I just crank the timer to 30 and let it go. It works just fine.