The x758 uses a K 92 series hydrostatic transaxle with hydraulic driven front axle. There is also a variable steering valve which adjusts the wheel speeds in turns that connects to the steering linkage. The steering valve on the front axle is there to speed up the outside wheel in turns and reduce scuffing the lawn. If the flow to the front axle is not correct it could cause problems making the front axle push or pull the tractor causing problems like you describe. On a new machine with low hours I would have the dealer check it out to make sure it is working properly.
I use the same hdap tires on my x749 and on an x758 I had for a while and neither machine had problems with soft ground.