I have found a 2002 manual BMW M3 for sale close to me for $7000 Canadian with 220000km. The car is in mint condition aside from one issue. The car stalls when put into neutral at highway speeds. The owner has not provided any other details other than a full walk around of the car idling for 5 minutes with no hiccups and showing no lights on the dash. The owner says this happens at highway speeds. The only modification that I can see is done to the car is an aftermarket K&N cold air intake. What might be the issue? Is the fix costly?