Pontiac G8 2008-2010 - 20" rims/tires causing a shake

Any ideas. Get a shake over 60. Not bad but noticeable. Gets worse at higher speeds. I have 20" rims and tires. Have had them road forced balanced. Readouts all positive on balance and tire uniformity. My 18" snow tires and rims ride so much smoother then my 20". Could it be that the 20" are to much for the car. They are forged 3 piece custom wheels. What gives? Almost make me want to put the snows back on. :bang:

These cars are very sensitive to the slightest balance issues. I would have the wheels rebalanced at a different shop. My G8 would shake really bad with a couple of different sets of wheels I had but not the stock wheels. I had the aftermarket wheels rebalanced and they were fine afterwards.