Thanks for your replies. While the engine was running I pulled a spark plug wire and nothing happened. I pulled another one and nothing happened. Then I pulled one more and the RPMs dropped. So I now know that at least two of my plugs/wires aren't sparking. I tried putting a different plug in but it made no difference. So it's either the wires (which are brand new) or maybe the distributor cap (which is only a few months old). So confused..