Is underside of deck clean? Have you removed your blades recently for cleaning, inspecting, sharpening, balancing, & reinstalled them per your manual for timing purposes? Have you greased mower & spindles recently? Check engine oil level on dipstick & make sure oil is on full mark before every start. Check air filter for replacement. Remove blower housing, top cover for engine, & clean engine cooling fins by hand, no water, I use a Shop Vac after loosening up dirt, crud, etc. with small tools & brushes. Just a few possibilities I came up with. Let us know how it goes, thanks, Mark