I've had two plumbers camera my sewer now, and gotten two opinions. Finally had a DVD made.
Plumber 1 suggested that the issue is a "massive belly" in the line (which the DVD does not seem to support).
Plumber 2 says there's a cracked pipe in the line, but I'm not sure of that either. He notes the collection of stuff around 3:45 in the video, but I'm not seeing a cracked pipe.
This is what he pointed out basically for the cracked pipe:
Both offered quotes to replace it for $8700 and 7200 respectively.
Noone seems to have a good guess as to what the gunk in the line is; one suggested mud, but wasnt sure. Dirt makes some sense as we have a floor drain in the concrete area outside the basement and its pretty dirty, but who knows - we only bought the house in September)
Camera on youtube:
Obviously I dont want to replace the pipe unless it's necessary and I'd love to have someone who isnt going to sell me a new sewer give an opinion or two.