my ford focus starts up but it acts like it is running out of gas

Sounds like your fuel pump may be bad or stopped up fuel filter

I had the same problem; it was the PCV valve. Cost $65 at my local mechanic, but if you can get under the car, it's behind the engine (I've got the DOHC Zetec).

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 !!!