Headlights dim, transmission jerks and car nearly dies.

Ford, Focus zts, 2000, 88000, 2L DOHC
My girlfriends car is having an intermittent problem and I could use some help with the diagnosis.

2000 FORD FOCUS zts
4 Cylinders, 2.0L SFI DOHC with an automatic trans. And AC
about 88,000 mi.

The problem is that the headlights dim. This problem is intermittent, inconsistant, and only happens while driving. On some nights the car is fine, but on other nights this is a serious issue. I'm afraid that one night she is going to end up getting stranded somewhere, or even worse, get into an accident.

Here is what happens: the headlights dim (as if they are going to die) and the transmission jerks; the car feels like it's about to die. Then the car returns to normal, as if nothing happened. This all happens in less than two seconds. But on evenings when this takes place, it will happens several times.

I thought it might be a failing alternator and had the charging system checked at autozone. It tested ok with the headlights on and off; but naturally we cant test it while driving.
I considered replacing the alternator just to see if that would fix the problem. Til I saw the price of an alternator for her car $$$. Needless to say, I didn't take that leap of faith.
We are desperate; if I don't figure out the problem soon then she's going to have to put it in the shop, and she can't afford that. Please help.