I was out mowing with my new eXmark FrontRunner this afternoon, which just turned over 10 hours on the meter. It uses the 60" shaft-drive Ultracut deck and is driven by a 25 HP Kohler. I was really impressed with the demo mower and the absence of vibration when the blades were engaged as opposed to my 60" Lazer Z with the same deck and engine. When my own FrontRunner was delivered, the blades fired up just as smooth and ran from idle to full throttle without a trace of vibration. Just fantastic. But then today while finishing up the yard (I mow about 4 acres), the deck began to resonate up and down with this irritating vibration for no reason whatsoever. The vibration transmitted through the floorplate and control levers. I immediately stopped the mower and inspected the underneath of the deck for possible problems. No loose shafts or broken blades. I also checked the top of the deck for any debris or obvious problems and watched the pulleys while the deck was engaged. The belt was vibrating and flapping something fierce and the tension pully and spring was bouncing back and forth in tune with the vibration. It almost acted as though a chunk was missing from the belt, although it was fully intact. My yard is flat and clear and I hit nothing with the blades to cause anything to bend or break. This mower's way too new and awfully expensive ($11,000) to be acting like this already. It's the weekend and my dealer is closed till Monday, so I thought I'd put this out there and see if anyone else might have had or is having a similar problem. I hate to continue using it in fear of causing the problem to worsen. It's bad enough that this deck is laden with weld splatter making it super difficult to scrape. And even worse that the lift-deck design allows the linkage to literally wedge and burry itself into the top of the deck leaving a nasty gouge in the steel. Now I feel like I just finished running a jack hammer for two hours. Any help would be greatly appreciated. Thanks!