Third time is the Charm...Machine Fatigue? Ron?

I'm trying to get going on a quilt promised to Gbebe. Last quilt I did, after five hours straight of random quilting on the quilt top, my machine began showing signs of stress. Stitches got shorter, and seemed to slow--although that may have been me slowing-- anyway, after a rest, the stitching was back to normal. I never could figure that one out. It wasn't the needle, as I managed to break a couple of needles during the process, so I was replacing them as I went along. Five hours almost non-stop just seems to be the limit for this machine, which is a 60's-70's Singer. I do know that industrial machines are made to run non-stop during a long workday, but they are also getting regular servicing during the running time, which leads me to wonder if I should stop in process and oil the machine. I do clean the batting fuzz out as I sew. Maybe this is a question which Ron can answer? Cea