I was mowing this afternoon, bone dry, moderate/low growth, cutting at 3.25". I'd been cutting as usual for about 20 minutes, up and down an 8-10% grade when the power suddenly dropped on the uphill trip. I don't have a tack but the engine was laboring and struggling. No noise that I recall, just an instant power loss. I idled down, turned off pto and engine. Let it sit for a moment and it fired right up. Sounded normal under no load and even cut fine on level ground/downhill but when put under load on the uphill the power cut down and the engine lugged, not to the point of dying, but certainly not the go thru anything power it had before. I finished the section I was mowing and put it up. The motor backfired sharply last night when I cut it off, I just figured that was because I didn't use it long and it didn't fully warm up, and then it backfired again sharply this afternoon when I started it under low throttle. Mower has about 35 hours. 2013 Bob-Cat Predator Pro with 1000cc DFI Kawasaki. I called the dealer and he had no ideas other than bring it in and let him test it. Anyone got any ideas what's happened? If it was an old 150 Mercury Black Max I'd guess I'd dropped a power pack.