I have a R120S articulating rider mower that was purchased last August. I mow on the height level of 3. All of a sudden, it is mowing low, mostly on the left (which is worse if the ground is not level). It mowed just fine until two weeks ago. My local authorized service center replaced both blades, changed the oil & filter ($245.30, of which $80 was travel), but it is still doing it. I have ridges and scalped areas all over the place. I guess I need to guide the service center. Where can I get a complete list of what might be wrong?