1996 Saturn SL2

I was driving down the freeway when suddenly my RPM gauge started going crazy, then my temp. Gauge, followed by my mileage gauge; after which my lights went dim. I continued to drive approx. 7-8 miles @ 55 MPH until I could get off of the road to turn the car off. When I got out of the car I discovered that it appeared to be very hot (but never read hot). The very next morning my engine then started malfunctioning; jerking and bouncing from one gear to another. I discovered that my transmission fluid level had suddenly rose extremely above the full line on the dip stick; however no additional fluids had been added. My car has been making strange chirping and squealing noises from the serpentine belt. I took my car into Auto Zone to have it tested and was told that the read-out indicated that the number four plug was misfiring. I immediately replaced the battery and plugs. The car showed improvement for almost five miles but then started doing the same things as before. Do you have any ideas as to what might be the problem and provide me with some directions to trouble-shoot? Thank you in advance for your time and attention. God bless.