I have a John Deere X304 mower that I bought new 10 years ago. I regularly mow 2-3 acres of hilly, some steep, uneven ground, sometimes through tall grass. I had to replace the transmission 2 years ago. The symptom then was it was loosing a lot of power and speed (but the engine itself continued to be running fine). The "loosing power and speed symptom" has returned for the last couple of months (which makes me wonder about the quality of my repair, which was done by a non-authorized mower repair shop). But my biggest problem is a new symptom that started just yesterday. The mower starts and mows fine for about 10 minutes (engine sounds good, runs evenly, mower works fine) until the engine gets hot, then it starts to run unevenly, sputters and dies if I leave the mower unit running. If I disengage the mower unit, the engine would continue to sputter but not die, but it continues to run unevenly. I am able to "limp" it back to the shed. After letting the mower cool down, the same thing happens - about 10 minutes of mowing. Air filter is fine. I have not replaced the spark plug or fuel filter, but could if either of them is considered to be the culprit.