Engine revs higher when I get off accelerator

Ive had this problem every summer the last few years when it gets hot. It sounds like a manual car when u clutch but didnt get off the accelerator fast enough. The revs go higher a little and stay there for a few seconds but the transmission is not engaged. It happens at all different speeds. And it starts at about 90 degrees. I changed the transmission oil and it still does it. It isnt getting worse, but every summer it does it. When I am on the gas, it works normal. No slippage. But when I get off it does it. Any ideas?