1999 Mercury Villager Engine misfire at low RPM

3.3 L engine seems to misfire at low RPMs only. It's not consistent. I've had the injectors cleaned, replaced the spark plugs and replaced the fuel filter. The injector cleaning seemed to help for a short period but it could have also been just a smooth period that I sometimes get. I think it might be the fuel pump but it seems to run better at mid to high rpms. Sitting at an intersection it misses but will smooth out if I put it in neutral and bring the rpms up to 2500. There are times when it will run smoothly so this puzzles me. Between the smooth times and the rough idle times it seems to be getting progressively worse and is now affecting low end speeds. 15 MPH or less. Once rpms come up it seems to be OK. I'm leaning toward the fuel pump and I think I'll replace it anyway. If that does not improve things I'll assume fuel injectors unless you have a better idea. I have inspected the plug wires, distributor parts. This seems like a fuel flow problem.

UPDATE: Thanks for the response from the team. However I found the problem. It was a defective vacuum line. I cut it a little shorter (I will replace it soon) and plugged it back in place. The hose connects to the end of the intake manifold and supplies vacuum to the cruise control.