No Check Engine light right? so it's not mis-firing.
I'd start with fuel pressure since it "changed" the symptom after the filter replacement.
You can sometimes borrow specialty tools from a lot of Auto Parts stores, call around and see if any lend out a fuel pressure gage. Connect it and turn the key to Run without cranking it. Do this several times and write the readings down. Then start it and measure the pressure again. Maybe run it up ~2000 rpms and check it too.
I'd also look all around the engine for a loose/broken vaccuum line. You may be able to hear it hissing with the engine running.
Edit: Verify that your spark plug wires are properly connected if it got even worse after replacing them. It's easy to put them in the wrong spot on the coilpack. How did the old spark plugs look and what type of replacement did you install (please say Motorcraft or Autolite)?
2000 2.5 MO5D 4.10 one-wheel drive DD - gave to my older son
1990 STX 87 2.9 going in.....Junked it
2000 3.0 4WD M05D 4.10 S-cab Off-Road package with 235-75-15" tires, hotrod DD, lol
Last edited by Earl43P; 07-28-2008 at 06:34 AM.