Car runs rough or cuts out when engine warms up

If your car starts perfect from a cold start then starts playing up when it reaches normal running temp, or if it even cuts out and then wont start again until the engine has cooled down then your going to need to get the following checked.

the most likely cause of your problem is going to be a bad oxygen sensor or lambda sensor for its proper name, now the reason it points towards this being the fault is that this part does not start working until the engine or exhaust is near normal running temp.
so you will find that your vehicle will start and run perfect when cold how ever as soon as the engine gets warm this part will start working and cause the car to cut out due to it sending the wrong info to the car fuel management system.

now you also need to bar in mind that this also depends on where your oxygen sensor is in your exhaust now if the sensor comes before the catalytic converter then the chances are your problem is going to be with this sensor, how ever if your sensor comes after the cat then it could also be the cat that is at fault.
if your cat comes before the sensor then it would be a good idea to have a diagnostic check done as you could spend money replacing the sensor and find out the fault it with the cat so bare this in mind when finding out the fualt