what cause an o/d light to start flashing on a 03 Ford Focus se ?

I have an '04 that's had the same problem except it was shifting hard from 1st to 2nd gear, also the electrical system would cut out and cause what I can describe as a stall. Voltage on alternator checked and found to be within range however the resistance was WAY too high for a low-voltage power system. Replaced entire negative battery wire/clamp and the battery itself. Ran a second ground from the negative battery terminal to the car's frame. Drove the car @ 45mph for 3 min without stopping (may be hard depending on where you live) to let it reset the oxygen sensors and whatnot. ALL problems have ceased. Good luck!