Could be a lot of things, getting the codes read for free at auto value might help identify a bad sensor or something.
Tune up is good?
Any Vacuum leaks?
Try spraying some carb cleaner around the edges of the intake and Vac hoses, if you're idle changes with the spray, it's leaking and will cause it to run like garbage.
If it has never run right maybe it has the chip programmed wrong or for another engine,
Howell has free support, Phone: (810) 765-5100