03 focus runs fine. Acts like it is starving for fuel when come to stop at light or something. Pump gas a little and takes off runs fine. No check engine light.

check battery voltage, it seems these cars have a shutdown condition when the voltage drops below a certain amount, and not holding peak voltage

Sometimes a battery pole is not tight causes exact same problem. Also replace fuel filter to rule out. Hope this helps.

I changed all these when I had that problem but can't tell you which one fixed it but i changed fuel pump,fuel filter, fuel pump module (under back seat on passenger side), throttle body (if you change this you must do a throttle body relearn procedure you can Google how to do this like I had to) mass air flow sensor, pcv valve and the two hoses that attach to it (the hoses get hot and crush in on themselves after time their garbage change them if you change pvc valve), up and down stream o2 sensors, and checked all vacuum hoses for leaks, Cadillac converter, and when I did all this nothing worked until until I changed the cat converter last but I found my ground terminal was loose so I tightened that up as well and after I installed that cat converter and tightened up the neg terminal on my battery and did the throttle body relearn car is purring like a kitten ho something here helps !!!