I want to say cracked vacuum line or bad spark plug wire. I know you said that you are not mechanically adept but with the car running try spaying some carb cleaner around the vac lines and see if this affects the motor idle. It seems that there is a vac module that controls idle/fast idle on these cars. The fact that it's getting progressively worse suggests something along these lines. Also because the engine was just fiddled with during your oil change may have exacerbated the condition. If you're lucky, you just have a crack on the end of a line which can be cut off and re-attached. I wouldn't run out and buy a bunch of parts just yet. Also, the fuel filter or even the fuel pump are suspect in my mind. For your own piece of mind, try taking out the air filter and see if it affects the condition. You have nothing to lose but a few minutes of your time.
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