1998 Other Volkswagen Models Car stalls

I have a 1998 Volkswagon Cabrio. It stalls after I've been driving it for about a half-hour. It has happened three times. The first two times, after I waited awhile (30 minutes?) the car started again and was drivable. I changed the airfilter and cleaned the throttle. I took the car to inspection (yesterday) and it passed. Today the car stalled again. (I'm waiting to see if it will work in an hour). It tends to stall when I am slowing down (turns, stop and go traffic) but not actually stopped. The battery is new (6 months) and I've cleaned the connections. I have plenty of gas and added an engine cleaner when I cleaned the throttle. Any suggestions? Thanks.