This car in spite of numerous tire replacements continues to shimmy on the highway. What’s up?
Out of alignment
Worn suspension components
Worn engine or tranny mounts
Tire replacements are usually accompanied by wheel alignment. Did you do that? If not, why not?
Pontiac. That doesn’t tell us too much. Year and model would help enormously,
While you did not see fit to share the model, model year or the odometer mileage of your Pontiac with us, my first thought is:
Numerous tire replacements? Within how many miles/years?
Since tires will usually last for several years, if you are replacing them very frequently, it certainly sounds like you have issues with wheel alignment and/or worn front end components. Since worn front end components can be a significant safety problem, I would suggest that you get this car to a reputable front end mechanic (NOT a chain operation!) in order for it to be checked.
What is now a shimmy could become loss of steering ability very suddenly, so this is not something that you can continue to ignore.