Toilet water/feces comes up into my bathtub w/o warning. This has happened ever since I bought my house a few years back. I was previously told that this was due to roots from a large tree in my front yard and I had put some kind of root killing chemical into my toilet, which seemed to work for about 1 year, but now the problem is happening again. It has occurred already 3 times this week. Do you have any advice? Please help. Thanks!

Roots seek the nearest source of water, in this case a break in your sewer line. That old sewer should be replaced by a new PVC one. First snake the sewer line and remove the roots.
For immediate relief and to kill the existing roots and prevent further growth you can purchase RootX or Robics Foaming Root Killer that contain Dichlobrnic.
A less expensive way would be to call around to garden supply stores and ask for fine grain Copper Sulphate. Put a 1/2 pound in your toilet and flush it down. Repeat in 6 months.
Let me say that sooner or later you're going to have to bite the bullet and replace that old sewer line with PVC.
Hope this helps. Tom