Get them balanced?
This is where i would start too and what spinkster said
Have you gone up to bigger tyres as well?
If it's not the balance then here's some other things i would suggest you check
Play in the steering ball joints, track rod ends, and UJ in the steering column. Shims and preload on the swivels. Steering damper
I had the same problem and it turned out to be the bushes i had fitted to the panhard rod weren't up to the job so i went back to Metalastic ones and this cured it.
HTH
