Have you tried dropping the tyre pressure a little ?
Get a low 0 - 40 psi tyre pressure gauge from a pushbike shop ( don't know why mower shopd don't bother to stock them )
You can also get them from farm machinery outlets if you are out of town.
You need one like this so you can make accurate adjustments,
Start at 14 psi then drop it in 2 psi incriments