2005 Chevy Malibu Engine idles down to quick

If you are driving about 40 mph and take your foot off the accelerator the car idles down immediately to about 700rpm, when you accelerate again the engine revs up about 500 rpm and engages the transmission. Not so smooth. The car engine runs fine, the transmission engages from a stop and shifts fine. I have changed the transmission fluid and filter recently but did not notice much change. Its not noticeable at a higher speed. What might cause that??