I have a laundromat with 16 washers and have the same symptoms. The entire four inch cast iron drain was installed new in 2003. Fortunately for me it is accessible from the floor below. I removed a section of pipe and was amazed to find that the pipe was restricted by almost half because silver quarters that had come through the soapy waste water were bonded to the cast iron pipe. To loosen the quarters it took a hammer and a chisel. It appears to happen in the first part of the drain where the waste water slows down and the quarters drop out and settle into the caustic environment and bond chemically. I am in the process of replacing the cast iron pipe with plastic ABS pipe.

Hi Dean and welcome to The Plumbing Page on AMHD.
You piggybacked on a 2 year old dead thread so I gave you one of your own. Before you post look in the upper left hand corner for the date.. This is first for me. I guess the smaller coins made it out into the city main.
Laundromats are fun to do. I've plumbed several in shopping centers in my area,
Thanks for your story. I hope plastic works out better the cast iron but if the quarters were to heavy to make it out of the cast iron trap wouldn't they also be too heavy to make it past a PVC one? Good luck, Tom