My niece has a 2005 Chevy Malibu (LS I believe) with a six cylinder engine. It has approximately 108,000 miles on it. It has new tires, a recent oil change, and a new battery. Generally, it rides very smoothly; however, occasionally, when she drives it for a number of miles, it will start to shake when she reaches 30+ miles per hour. She has taken it to a local mechanic, who drove it on a longer test drive, and the mechanic cannot replicate the issue. The check engine light is not on.

Does anyone have any experience with the cause of and solution to this problem?

Thanks