I bought a used exmark 52" about 3 mnths ago and its been running great until last week. I cut a property and it ran fine. Went 2 houses down fired it up and it sounded strange like it was not running at full throttle. When I engaged the blade it almost cut off but slowly got back up to full rpm's. I took it home and checked the air cleaner and plugs and one of the plugs was loose I tightened it up and it ran fine for a couple of days and it started doing it again. I checked the plugs and they were tight. Took it home and started it up and after about a minute I heard the other cylinder kick in and I engaged the blades and it ran fine. Has anyone had this problem or can someone shed some light on what this might be. Its a 2000 model with a 17.5 horse kawasaki.