Warren:--If the sewer camera had a counter on it, and you can determine approximately where the trouble spot is, It would be far more cost effective to make a sectional replacement of the sewer than to replace the whole line or continue to have stoppages cleared. You may find that the failed area has a belly in it (a low spot). This condition will cause both stoppages and premature failure of the cast iron. It is also possible that the pipe is just off grade and has a belly in that section. I use a sewer camera in my work, and I frankly find it very difficult to make a good call on cast iron condition, especially the invert.
There are means of relining sewer pipes, but I have not seen any for small bore piping, most of this insitu type repair is for municipal sanitary drains that are located beneath streets or areas that are conjested with other utility piping in the same area. It is quite expensive but a lot cheaper than digging up the road way to make repairs. Lots of luck and Merry Christmas..Bud