2002 Ford Taurus PROBLEMS STALLING WHILE DRIVING

I started having problems about a month ago - I left with the car as I was going around the block and it was like it had no power and it was like it was going to stall. I barely made it to the garage a few blocks away. Powering out like going to stall.

The suggested I change the gas filter. That seemed to fix - or so I thought.

Then I was going through and intersection first thing in the morning, and it stalled as I was driving. Had to be pushed to the side of the road. WOuld not re start.

THen tried it and did start - went right to a garage and they tested battery fine - they said my alternator had to be replaced. So 400.00 later.

Less than a week later I had the same problem. Back to Canadian Tire - tested again - battery not good.

A couple of days ago - the same thing happened. So the same garage can find nothing wrong.

What the heck is the problem?

Thanks