How many miles are on it and what have you done so far to correct it? Not knowing much about the car, I'd start with the basics;
Check to see if there are any codes.
Do a good basic tune up.
Have the injectors cleaned.
Check the fuel pressure and flow rate.
Check the timing.
Install a fresh set of oxygen sensors.
Clean the throttle body.
Pull the spark plugs, check for lean/rich condition.
Look for vacuum leaks.
Install new air, fuel filters.
Intsall new spark plugs, dist cap, rotor, wire set.
Check for clogged exhaust system.
Dump the codes and see what the computer says is wrong…
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.
IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.
Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.
For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.
Do the basics first. Dump the codes, fix any that you find.
Set the timing with a timing light, don't do it by ear. You will never get it right. Use 10-14 degrees advance depending on the octane requirements of your car and how much $$$ you are willing to spend for gas.
Don't throw parts at the problem, think, plan, test, ask questions if you don't know and then develop an action plan.