Less than 15 minutes
This fix WILL cause your Check Engine Light (CEL) to come on, and this hack is certainly considered messing with your emissions control system. In reality you won't get in any trouble for doing it, and the CEL is annoying but it can be disabled as well (take out your instrument cluster and remove the light bulb for it) Use the IndiGlo Gauge or Indiglo odometer hack to help you in getting that far if you want.
1) Locate the EGR (Exhaust Gas Recirc) Valve. It will be a Circular metal object near the throttle body behind the large air intake tube coming out of the airbox. It will have a Black rubber tube coming out of the top of it, and may resemble a UFO :)2) Yank that sucker off! (The Black Rubber tube, that is. Pull The tube off the top of the EGR Valve. To keep this hack from depressurizing all the rest of the things that require vacuum, shove the golf tee into the hose and if you want, clamp it down with a small hose clamp or nylon zip ties.
3) Test it out. See if it works. It may take only a few seconds for your Check engine light to come on. Mine took a few days though.Why it works?! For some reason, the PCM on a lot of Foci seems to like to modulate the opening and closing of the EGR for emissions purposes. For most people, it is more noticeable in mid range RPM's on level ground, at a constant speed, like going 45 in 3rd gear. The "fluttering" seems to stop when the throttle is wide open (flooring the gas pedal) and that's because the PCM shuts down the Emissions controls under high load, in a similar fashion to how the Air Conditioner compressor shuts down as well.
What this hack does is makes it physically impossible for the system to open the EGR Valve, even if it's trying. It will eventually sense that the EGR is not opening when it commands it to do so, and after repeated errors are recorded, it will be flagged as a "fault" and that's when you get the Check Engine Light.To clear the CEL, simply unhook the Negative battery Terminal for about half an hour. To completely un-do this mod, reset the CEL as stated above, and place the EGR Vaccuum hose back on the EGR.
If the stumbling stops after doing this, then you likely need a new DPFE sensor, or a chip that disables the EGR system entirely.