Why does my 2002 gmc safari van run rough or wont start after sitting for month or soi have changed wires coil distributor cap rotor and still it happened 3 mo later?

Are you using any gas stabilizer while the van sits? Also are you leaving the van with a full tank to prevent condensation? If the tank is not fuel, condensation can form inside and that turns into water in your gas. USe an additive liek Stabil when you park the van and make sure the tank is topped off. Gas will deteriorate over time, just how much time has a lot to do with conditions.