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The reason the car usually shut off is the the crank shaft sensor is not sending a clear signal to the ECM engine control modual. the sensor is located on the engine block next to the oil filter and that area is rather hot casing the sensor to overheat and go faulty. Replacement should rectify the problem.
This should answer your question but should you need more information do not hesitate to ask.
On a car of this age, the first thing you need to check is the timing belt. Remove the engine cover, uncle the timing belt cover 2 clips, and pull it slightly to the side. Make a mark on the belt with a pen, marker, or crsyon, and try turning it over again. Check to see if your mark moved. Usually when the belt goes, it does not actually break, but the teeth at the lower end shear off. The engine will usually spin over faster, like it has no compression, but I had one recently that seemed to sound normal, so a physical check of the belt is the first step.
Gomi might have a point depending on how up to date you have kept the service's. Please do not hesitate to ask if you do not understand what is going on.
The only way you will find this problem is if you connect a VAG-COM scanner tool to check all the sensors to see their actual signals. It is very hard to tell of hand what cause this behavior I have found many problems this way with the scanner tool. It usually is the crank shaft sensor but there can be other causes. I very hard to tell without the scanner tool. I hope you will understand.