Martin T - 4/3/2006 3:19 PM
First: check the vacuum lines in the engine compartment. If it started right after changing oil, you might have loosend a line unintended.
This is the ticket. Anything else is uncanny coincidence. Locking the master valve is allowing enough cumulative pressure in the entire vacuum system to make up for what you unplugged in the motor bay when you did the oil change, allowing the car to shut-off. This happend to me just yesterday, and common because of the line arrangements.