Engine runs rough and backfires under throttle

Hi. My Ryobi mower (Kohler Courage 19 hp) runs really rough and sputters/backfires hard under load. First season doing this. Cleaned the carb 2x, new air filter, plug, and the gas is new. Fuel solenoid seems fine. Fuel supply seems fine. What's weird is I thought I had the problem fixed (after the 2nd carb cleaning) but during mowing, after about 30 minutes of use, I turned a corner under normal mowing conditions and the problem immediately returned as if a switch had been thrown. Now the engine also runs rough/sputters/surges under all throttle settings. I get a little oil in the exhaust at startup (been that way for a couple seasons). Other than that, the mower has runs fine for 5+ years. Any help would be appreciated. Thanks for your time.