tranny or electrical issues?

Have a 2002 pontiac sunfire and have been having the tranny slipping, the speedometer spiking and the radio cutting out all in the same time. Used to start around 40mph but on the interstate it slipped very badly and seemed like it couldnt decide between overdrive and 4th or 3rd (whichever one, I dont know). This happend for 4 hours of driving w/out any signs of letting up. Now problem begins around 20mph and persists till the I park the car. Also the drivers side headlight is very dim and have replaced the bulb twice but all with the same amount of dimness. Engine light and service light are both on but shops say they cant read the car to figure out the problem. Any ideas would be GREATLY APPRECIATED!!! Thanks

I think you have an electrical problem that probably relates to the ground wiring somewhere in the system. I would check the main ground coming from the battery and then the smaller grounds. Check every ground you can find starting at the battery including the terminal. Your transmission is electrically controlled and if the ground contact is fluctuating then your transmission shifting may be intermittant and causing slippage as the solenoids don't know when to properly engage.