2007 Kia Rio car reves from 500rmp up to 2000rmp after gasi

Every time I put gas in my car and it doesn't matter how much I pit in I will start to reve from 500- 2000rpm when I start it up after putting gas in the tank and the check engine light isn't on. And when I let the clutch out it would stall. Once you get the car moving and I push the clutch in it would stall. After driving for 5mins it would go back to normal. But since this has started 3 months ago my gas mileage has gone from an average of 42mpg down to 32mpg and I took to the dealership they have put on a new canaster and close off value. And the problem was still there. So when I brought it back fot the same problem last week the put on a new gas tank. Now my problem has gotten worse and they cant find any thing when they check computer they cant find anything wrong with the car. And since they replaced my gas tank my car has lack of power. At times when i'm on the highway my car would only do 80kmh or up 100kmh and now im getting 24mpg.
what would be causing this problem?