Sews for 15 minutes, shut off for 15 min, then...

Question

My sewing machine will sew for 15 min then quits. I will shut it oft for 15 min and then turn it back on, then it will sew for another 15 minutes. Any ideas or where I can get parts, because I love this machine, my grandmother gave it to me before she died.
Thanks,
Terri