Ford Focus ztw 2003 engine light

My car died yesterday. I got jump started took car for check up no problem with starter, battery or alternator. Drove to Dealer because while driving it with or without a/c on car will jolt and loose rpm (will significanly slow down to almost die, but it did not die!) Took it to Ford Dealer, they did not find anything from reading the codes or test drive. I last had a tune up in August 2008. (today June 17, 09). I am puzzled please if you can offer me some feed back. I drive quite a bit and long distances in rural areas. Don’t want to be stranded but it is my only car. What could it be? Thanks!!