[87 GT MUSTANG]

i was wondering if someone could help me out here, i have a problem with my engine idling up to 1500-2000 rpm's when i am at a stop light or just sitting in neutral, it is mostly when my car is warm that it wants to do this, it has been an on going problem since i first bought the car last year and i recently did a tune up including plugs (bosch platinum), wires (accell street custom fit), cap & rotor (platinum gold) and an oil change with valvoline higher mileage engine oil and it seemed to not want to idle as much as it used to but it is still there, i have been told to replace my oxygen sensors and i have been underneath the car and they look like they have never been done since it first came out of the factory, so i am wondering if anybody else has had this problem and if they have fixed it with the solution i just said (oxygen sensors) i know it would be a good idea to replace the sensors as it is but with $60.00 a piece canadian and with two of them needed, money is kinda tight, so that is why i need to know if this is the problem, thanx for your concern.