time to call the rota-rooter man. your drain for the basement toilet is the water source for a tree or shrubs nearby. the drain line is most likely clay pipe, which is noted for allosing roots to enter around the joints. your final solution is to dig out the line and replace with 20 foot length pvc carefully glued at the joints to keep roots out. wrapping 4 or 5 turns of 12 gague copper wire around the joints will discourage roots as copper is poison to most plants. the digging can replace jogging as exercise for you for a few days :o)