My 2000 ZX3 had quite a bad surging issue, as does practically every non-SVT Zetec Focus. The DPFE and EGR system is the culprit the majority of the time.

Before you do anything, yank the vacuum hose out of the top of the EGR. Plug the hose so that you won't have a vacuum leak. Your check engine light will come on, but this may cure the issue. Drive it around for a bit over the next couple days. If the stumble is gone, then the DPFE sensor is most likely bad. I think they are still only like $10-15 new from Ford.

I also found that regardless of the status of the DPFE sensor's functionality, that my car still had a partial throttle stumble. Putting a vacuum reducer onto the vacuum hose coming out of the EGR helped quite a bit on my car. I'll try to find some more info on this for you.

You can also get a custom tune to disable the system all together...but obviously that is the more expensive route.