Stalling when we are driving on the highway?

Hi,
Our car has started stalling when we are driving on the highway. The first time it stalled we were on an exit ramp. Now it has started doing it while on the highway on an incline using cruise control as well. The problem was at first intermittant, but now is fairly frequent. We took it into the dealership and they ran a scan and could not locate any issues. Per thier suggestion that they had seen this issue before they replaced the Ignition. The problem started up again the 3rd time we drove the car. Any suggestions? There does seem to be a correlation to the transmission down shifting and the stalling occuring, but the dealership did not think so.

Thank you!