I have two new ZD331's. Both have about 30 hours on them. One of the units will run like a scalded dog. The other runs great for about 30 minutes or so and once it warms up it feels like it goes from a 31 hp unit to a 21 hp unit. I am carrying it back to dealer again on Tuesday to see if they can get it right. Has anyone on here had a similar type problem with these mowers?