On many new cars the brake light switch is linked into engine management as because when you are braking the fuel mixture can be weakened. sounds like a trip to dealers im afraid.
Daft question, but have you checked your brake lights?
We had an Astra in the other week for the same thing, one of the brake light bulbs had blown, and caused the fault.
Going to a dealer is an option, but they can be quite expensive for even a simple diagnostic check.
Any decent independant garage with the right equipment can plug into the car and tell you exactly the cause of your problem.