Ford Fiesta chugging and stalling in an automatic.

I know some aspects of the problem have been posted on this site but I'm not sure whether the answers are applicable to my fiesta. I have always thought that automatics couldn't stall, but this wretched thing does. It seems to do it when the rev counter is at about 700-750, it either chuggs or just dies on me at traffic lights. I also have the problem of having to reverse at speed otherwise it dies again - you can imagine what it's like to reverse park in an on-street parking bay!! I have recently taken the car to a garage and they put it on the computer but nothing registered as a problem. I hope someone can help.