Hi, I have a similar problem with my villager. To be honest I don't think its worth putting any money into.
What seems to be happening is this, there is a problem with the AC system, that causes the AC compressor & fan to cycle on and off, the energy needed to get all that moving momentarily robs the motor of current ( bad spark @ the plugs). This causes a rough idle for a few moments, but seeing as the AC is constantly going on and off, it seems like the engine is always idling rough. I simply avoid using the AC, or use it only when the van is moving. Im having no problems.
Before spending money on anything else, Id have the AC looked @ first. I think whatever regulates the AC fan/compressor is bad