[lnfletcher]

I had the same problem!
I suggest one of you new tires is not round. They are balanced, but not round.
I struggled with it for years! Road force balanced by 2 different shops and they swore it was right.
Finally found the right tech and he found one of my rims was slightly out. I thought the road balance machines should show this but it didn't on my case.
Fat tires on a lightweight car, they seem to be more sensitive and the normal tech thinks its within tolerance!