My 2003 navigator wont go over 20 mph, what's wrong with it?

My navigator was acceleratinging up to 20 mph, then wouldn't go higher, then i would take my foot off the gas and push it back down and it would go up to 30 mph and so on, and so forth, someone told me it was my cad converters, then i was told that because smoke is blowing out of my exhaust that it probably just needed a tune up and oil change, which is correct? Please help!!