Starting
 

I have a starting issue where sometimes when starting the car it will crank and when you let off the key it seems to diesel or chug a little bit before either catching and running fine or stalling, i have changed the plugs, wires, air filter, fuel filter, emission lines,(not changed for this reason but cpu was throwing a code). Now I'm under the understanding that it can't be a coil because the coil is either good or bad, it doesn't just kinda work it does or doesn't is this right???? And I don't think it anything with the starter or flywheel because when it does fire up it works just fine, my only other concern is the timing chain, when is the recommeded change, how hard is it, and could this problem be caused by slack in the chain over time? Thanks to any who can help.