1978 Lincoln Continental Car shifts late and already change

My 1978 Lincoln Continental does not shift properly. It shifts late and sometimes goes in and out of gears between 60 and 70 mph. Sometimes I have to let off the gas to get it to shift. I have had the carburator rebuilt the vacuum modulator replaced and adjusted. This happens all the time regardless of cold or hot. While standing behind the car in park while it is running you can hear a slight "poppping" noise. It isn't loud at all but can be noticed when listening for it. The car currently shifts at 35mph then again at 60mph and then at 70 mph. It used to shift at 25mph then 40 mph then about 60 mph. There was one time when I was trying to stop at a stop light, the car just took off and gunned it without me touching the gas. Please let me know what you think. Thank you!