2002 Saturn SL1 RPM shoots up when decelerating

While driving at a cruising speed, the engine is about 3000 rpms. When I take my foot off the gas pedal, to slow down or put my foot on the brake to come to red light or exit the freeway, the rpm gauge starts to decrease to about 1900 rpm or idle, but then mysteriously shoots back up to 3000. Also, when parking the car, I will put the gear from drive to park and the rpm will shoot to 3000 rpm and just sit there revving itself. This happens sometimes. When in park and the car is having this episode, I have pulled the gas pedal towards me to see if it's stuck, and it's not. I have also stepped on the gas pedal to try to see if the rpm comes down, sometimes yes, sometimes no. Turning the car off stops the wild revving.

Gas mileage dropped from 35 to 29 consistently for 18 months now.

First, the check engine light came on and mpg dropped. Changing the front O2 sensor turned the light off, but mileage never came back.

Sine then, in my effort to return to my 35 mpg, here is what I've replaced:

Intake manifold gasket by mechanic (passed carb cleaner test 6 months later)
fuel filter (wix) (summer o8)
spark plug and wires (each 2x months apart)
1st pulled Autolites, put in Delco, the put in Autolite
cleaned EGR a few times with kerosene and carb cleaner, replaced egr gasket
cleaned out throttle body (Richpin06 youtube video)
replaced pcv valve (summer 08)
thermostat and coolant drain (april 09, engine runs hotter now)
engine coolant temp sensor (april 09, no different)
trans oil and filter (jan 09, no leaks)
air filter changed once a year
I have to put 1 quart oil in engine every 2000 miles.

Why is the mileage 27-29 mpg? It used to consistently get 35-37.