1999 Dodge Caravan 3.8 L engine idles rough and backfires at a certain rpm does not do it all the time any suggestions would be greatly appreciated?

Sounds like the van is misfiring on one of the cylinders. Best advice, take it to a garage and ask for an inspection, if you live in a state that has mandatory yearly inspections you could ask the mechanic during the inspection is it is misfiring. That or spark plug wires, but that's kind of a hit or miss. Hope this helps.