I am also having ignition problems like above. It manifests a little differently: no highway surging, but sometimes rough starting, definitely worse MPG, and less overall power. I have a 2001 Astro with about 52,000 miles.
In the last year I've changed out the cap & rotor twice. Both of the caps had brass electrodes. I replaced the cap/rotor at the end of November 2008. At that time I removed the screens in the bottom of the distributor, per the instructions that came with the new cap. About 1 month later, I opened it up again to find more of a greenish corrosion on the cap electrodes. The rotor tip was fairly clean, but I carefully sanded it by hand to remove any paint on it (not enough to remove metal). I also sanded the cylindrical electrodes to clean them off. I also vacuumed out the distributor, cap, and rotor to remove any dust, etc. I also at this time replaced the PVC valve, also suggested by the information that came with the new rotor.
The van ran well for a week or so, but it has steadily gotten worse in the three short weeks since. I've had the van since new. It used to turn in about 20mpg in town (I use a light foot), but now it's getting about 15mpg. All told, I'm on my 4th cap & rotor, including the original. The second replacement, done by a mechanic, was a cheapie with aluminum electrodes. I think the original had aluminum too. The original was in really poor shape. I have not opened it up since this last cleaning (it's too cold outside!). The check engine light is not on. Other than new cap/rotor, it has original plugs & wires (plugs looked good when I checked them last spring), and most everything else is original.
Could this constant failure of the cap/rotor be caused by a weak spark from the coil? Any other ideas?
I would really appreciate any help.