I'm assuming it's a petrol, not a diesel.
Could be one of several things.
Common one is lack of servicing. If the air filter is clogged it won't help. If the spark plugs have done 40,000 miles or more then they could be due for renewal. However if the car has been serviced properly and recently then common problems include.
The transponder ring which surrounds the ignition key hole. Could need a new one.
Another is the crankshaft position sensor. If that is dirty you could try cleaning it, or you may need a new one. They are not expensive.
However, it could be something else instead. You haven't described the symptoms, or typical occasions when it happens.
ChicksFan, November 2011
