Q: I have a 2000 Ford Taurus. A couple weeks ago I was driving and I lost all electrical power to it. Now it will run but very badly.

I have a 2000 Ford Taurus. A cpl weeks ago I was driving and I lost all electrical power to it No headlights/tail lights/emergency lights, no dash lights, no defrost/heat, no radio, windows were not woring But the car was still running, barely, pedal to the medal & was moving to the pace of snail. I made it, about a quater mile, to a public parking lot and shut it off. Waited for a tow. My fiancée immediately said it was the alternator so he changed it. It still did the same thing. So he replaced the battery, it did the same thing, and yes we've had them tested and have checked the tension on the serpentine belt & it's good. The new alternator and battery are testing good, but it still continues to do the same thing. What is my step/steps???

My car has 200000 miles.
My car has an automatic transmission.