My house is the same age as me. It needs help. Won't go in to the whole deal, but, I feel like the house has settled over the years and reduced the slope of my main sewer pipe under the house. I've seen a video of the inside of the pipe from my end to the street and there is nothing wrong with the pipe at all.
Needless to say, it is cast iron, and therefore no modification to that pipe is realistic. What I'm looking at is re-plumbing the entire house. My wife told me last night that she didn't care how much it cost, get it done. I'll have the last laugh there. I will not enjoy it though.