I own two 2000 Ford Focus ZX3's (don't laugh they were perfect for my wife and I while we were going through college) and they both were plagued by the same stumbling problem! Our cars would act as if they were running out of gas when the fuel gauge was reading just under a quarter tank. Instead of replacing the fuel pump I decided to clean the tank, pump, and pump sock. After removing the tank I cleaned the very filthy fuel pump sock with B-12 and rinsed the tank out using a towel and B-12. The cleaning along with a new fuel filter solved the problem for both cars! Total cost roughly $14, the cost of 2 fuel filters from AutoZone. I would recommend trying to clean the tank & a fuel-filter change before getting a replacement pump. If it doesn't work you're only out of a little time a little money; if it does work you just saved yourself several hundred dollars!
The problem seems to be that the fuel pump sits inside a "coffee-can" contraption inside the fuel-tank (this might act as some type of baffle in cornering?). My theory is that once sediment from fuel gets inside this "coffee-can" it simply deposits in the "coffee-can" and doesn't have the oportunity to flush around the rest of the tank. Since the sediment collects in a small space problems arise much quicker since the fuel pump is always drawing fuel from inside the "coffee-can" where the sediment is deposting. This is only my theory but judging by the deposits I witnessed, on both cars, on the sock and inside the can and how relatively clean the rest of the tanks were, it makes sense to me.
Remember: Try not to run your car with less than a quarter tank of gas.
If you have any questions let me know.
jason o'brien
2000 ZX3 Kona
2000ZX3