Our 2003 Dodge Caravan, V6 3.3 litre, Problem: Multiple Cylinder Misfire.

Background: This van was sporadically acting up, running very rough at times, then it would stop acting up. Over several months it did this. About 2 months ago the van acted up where I had to pull over, and I had to have it towed to a garage. The garage changed the spark plugs, spark plug wires and the fuel filter. It ran beautifully for about 2 months, and then it started acting up again, and the code says "multiple cylinder misfire". So it's doing the same thing again. What could the issue be, has this happened to anyone else?