Auto Parts: starting problem, coolant temp, crank sensor

Question
I have 1996 Accent . It has a mind of its own for starting
For months no problem and then wont start. sometimes  I wait for a day or two, then it would start
The engine is cranking pretty goog but wont start
I alredy did tune up , battery change
Somebody mentiond about changing crank sensor,
Pl. let me know if this would help and where is it located
Thanks

Answer
    I assume that the tune-up you did included new spark plugs.  Although you are probably going to have to get someone to hook you up to a diagnostic computer, you might try changing the coolant temp sensor.  Also dump a bottle of injector cleaner in the tank.