Car cuts off at red lights

I drive a '99 Volkswagen Cabrio.  It has about 72,000 miles on it, I've had the car for about 2-3 months.  The car overall works well in my opinion.  But, once a month while at a red light the car will just cut off, like I had removed the key from the ignition or something.  If I let it sit for a second, then put it in park it will restart.  Where should I start looking for problems?  I'd like to have a little bit of an idea what to speak to the mechanic about, or if there is any thing I can look for myself.  I appreciate any ideas, please let me know if more information is needed. 

thanks