A major problem with clay sewer pipe (besides its fragility) is its vulnerability to root intrusion.
In my opinion, once you have a problem with root intrusion you have only two good permanent options: 1) kill the invaders (with a root herbicide and/or clearing the offending trees from the property); or, 2) replacing the vulnerable clay line with something much more impervious to root invasion. In my opinion, the most impervious replacement is polyethelene pipe, preferably seamless, installed via the “trenchless” method.