On later cars (1996 onwards)the computer monitors misfires and flags witch cylinders are misfiring and a scan tool will identify them reading data straight off a tool. On an older car you have to start with the basics. If an engine is running poorly and you are told it was recently "tuned up", you must then look at the basics. An engine misfire can be caused by engine mechanical isues, fuel issues, or ignition issues. You've got to start with the basics, check the engine is mechanically sound (good even compression, no vacuum leaks,valves are properly adjusted (in the case of shim or screw adjusted valve clearances), check the engine's ignition system is good (good spark plugs coils or plug wires where applicable) then the fuel system needs to be tested and inspected to make sure it is operating efficiently. A misfire can be caused by many things but basic diagnostic tests need to be done rather than just trying to guess what the problem is. A mis fire will eventually damage the catalytic converter. I'm not sure what the blue book value of your car is look at www.kbb.com also think of what it would cost to replace your car with a comparable car and think do you repair or replace.