highway problem
 

I just bought 2001 Mazda Protege 2.0 LX from a car auction.It has 138000 miles, starts and dles good. I have two issues with this car:
1. I had a CEL with codes for misfire and the Catylic converter . I used gas treatment and changed the spark plugs, the light went away then came back after a week. I smell gasoline around the the back of the car.

2. When I drive the car,about 35 MPH the RPM will go up by itself to 5 or 6, then when I let go of the gas the car either shift or jerk then it will be smooth again for a little bit then it will do it again. On the freeway above 50 MPH the same thing happens every few seconds . It has an automatic transmission, the fluid is full.

A mechanic shop looked at it and I have two opinions from the same shop. One opinion is bad transmission, the other opinion is bad Catylic convertor because of the CLE code. I will appreciate any input or advise. Thank you and happy holiday.