Audi A4 2008 Not Starting Properly

When i go to start my Audi, if im lucky and catch it at a good time it will start
with no problem, which is why i dont believe it is a Starter issue but maybe
and issue with the flywheel? ,but other times ill go to start it, it makes a
horrible grinding noise and wont turn over, so i take the key out and try
again, sometimes it works ,and then sometimes i have to press the gas to
get it going. A few months back someone changed the spark plug and coil
but only on cylinder 2, the other three in there are old, im not sure if that
would may be the cause of the problem, the car is almost at 100,000 miles
so someone mentioned maybe the turbo,fuel pump or the timing belt but i
dont see how that could relate to it not starting up properly. (Also had the
battery tested and its not the battery.) I also feel like im losing some power
with the engine, feels like i have to press the gas a bit more than normal but
it is also due for an oil change. Anyone have any clue what it could be?