2001 Mitsubishi Diamante

2001 Mitsubishi Diamante. We have had it about six months. I have had it in to a very reputable service station and have come up empty on the following problem.

The car has new tires, this occurred before and after they were installed. During more aggressive turning the car gets very unstable, bad enough to lose control. The service shop checked out all the obvious front end stuff including the three bushings on each front wheel assembly. For the most part the car tracks straight down the road and is smooth and true. It may be described as a little soft in handling but nothing severe until this situation occurs. The shocks and struts seem OK. Does anyone have any thought about this car with 70,000 miles?