timing belt+ fuel injector

RPM shaking around could also mean timing belt is degrading. how much mileage is on this car? did the timing belt ever get replaced on 60K mileage maintenance? i guess probably not.
the reason to say this, is the common sense that the dealer should already looked thru the common places. like filter, spark plug, air intake, etc.
another is the fuel injector nozzle going bad.
$500 cost.