RPM needle goes up and down when stopped, engine shakes, car dies on 2001 Lincoln Navigator

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
2002 Lincoln Nav. 76,000 miles
The service engine light recently came on, but the car was running fine. I thought it waa a factory setting for when the car hit the 75,000 mile mark. A few weeks later, I was stopped at a stop light, and the RPM's needle repeatedly jumped up and down when I wasn't moving; the engine shook,as it idled between high and low. The power sounded different as I drove. At the next stop light, the car died. It restarted, and I barely made it the last block home; the car sounded like it would die any second. Any idea what this might be?
(1) Answer
driving with the light on for weeks is not a good idea. the problem may be compounded by that. you need to go to a shop and get this looked at before major problems develop from minor problems.