Sounds like you've got under-engineered wipers, not built for the forces that can happen in highway driving. You should make sure the motor/gear mount isn't being twisted by the wind forces. Also make sure the wiper arm hubs aren't shifting on the shaft.(try making indexing marks to detect shifting) If they are just not up to the job, check out wiper systems used on interstate buses. The other alternative is to park when the winds and rains get too much.
Many years ago I drove a bus that had vacuum operated wipers. When going up a hill, accelerating, or fighting a head wind, they would stop due to lack of vacuum. You had to either let up on the gas pedal to get a wipe or two, not always easy to do when going up Pennsy grades. We finally added an electric vacuum pump to keep them moving.