2003 Chevy Venture Transmission slipping when starting off

I recently had the transmission filter and fluid changed at my local car repair shop. It is not a transmission shop. Shortly after picking up the van the transmission started to slip whenever you come to a complete stop and began to accelerate from the stop. It does not do it everytime or with any kind of pattern. It will also do it in reverse sparatically. I also have a 2003 Buick Century with 110,000 miles that this same shop serviced the transmission by changing the filter and fluid approx. 4 months ago. The car started do the exact same thing immediately after being serviced. At the time I just thought it might be because the car high miles. The transmission in both vehicles use the same tranny filter. Do you have any suggestions or have you ever heard of this happening?