We are getting a misfire error code on our 96 Dodge Caravan - 306,000 mls. The distributor, plugs, and wires are all less than one year old. It only seem to happen if we drive on the highway for a long distance and when it happens, we really don't feel anything different on the engine.

What can cause this?

 

I am guessing you have a 3.0L motor. Can you pull codes and post them, like p0300 or p0304. That would help. If you have a 3.3 or 3.8L motor, disconnect the knock sensor next the oil filter and starter. This sensor will cause a misfire code. There is a TSB for the knock sensor.