2002 Mercury Sable • 100,650 miles

My engine light came on and stays on. The car starts, and sometimes it seems like it's running fine, then all of a sudden it will run really rough, like it wants to stall out but it hasn't yet. Also, I noticed that the lights will all of a sudden start to dim and brighten like they are flickering. Any ideas what could be wrong? I am going to get it fixed, but I wanted at least a heads up as to what I might expect. Thanks.
February 3, 2013.

Take the car to a nationally recognized parts store and have them scan the computer for trouble codes. Most will do it for free, and it should tell you where the problem is coming from. If you have further questions, let me know what the codes are that are found.