My microwave does not stay on steadily, but seems to pulse up and down?

My microwave does not stay on steadily, but seems to pulse up and down. It is a huge, 25-year+ old Tappan free standing. I don't remember if it has always been like this or if it stayed on steadily before. Any insight?
Linda, July 2015
hi linda,i think its beter to replace the microwave with a new one part aint available anymore for your old one

francois, July 2015
link Click here to see other fixes for Tappan.