I have an exmark Viking 48" hydrostatic 2013 with 80 hrs on it and am having a lot of problems with the cut. My mower is leaving a lot of areas of uncut grass. I have tried all different types of blades from mulching to medium lift to high lift with all having the same problem. I have cut in wet conditions and dry and all with the same result. I can go from speeds of 1 to 6. When I go at 2 it cuts great. However i usually go at 4 which is not too fast, and I have seen a lot of uncut areas, requiring me to double cut. Is it this particular model that is having these types of problems or do I need to upgrade to the next model of walk behinds. Please help