2006 Grand Prix 3.8 Base

Symptoms: Doing 75 mph on open freeway when steering wheel began to shutter and car started slowing down like the brakes were being applied. I put foot on brake pedal and any pressure at all felt like brakes would lock up. There was no play at all in brake pedal. Pulled off freeway and could smell brakes. Front rims on both sides were hot to the touch. Back hubs were normal. Limped to a nearby Pep Boys shop. They could touch the car for a couple of hours. When they did, they wheels turned freely, they checked front rotors and brake pads to be fine, test drove it at 40 mph and told me they could not duplicate the issue.

Today, same scenario, 75 mph, steering wheel shuttering, I applied a very slight brake pressure and the shuttering got profoundly worse. Then, put my left foot under the brake pedal to pull it up. I didn't sense any movement. I pulled off the freeway and pumped the brakes several times. Started driving again without the issue. I then drove the car a Pep Boys near my home that had put on new front rotors, front brakes and alignment 700 miles ago. I had bought the car with 34,000 miles on it last month. I have driven it a total of 700 miles. So, can't say if it is an existing condition or a new problem.

I looked up potential causes and the ABS wheel sensors could be a problem. But, so could several other factors. I don't think that Pep Boys mechanic can track this down without duplicating the problem.
No ABS warning lights have come on. And, I did not check for any codes.

Has anyone experienced this type of braking problem and found the solution?