Your engine is in need of the valve guide/valve seal treatment pronto. The roughness and "kicking back" are related to plug fouling. If you want to really get a true view of the problem. Change the plugs, drive 200-300 miles and take another look at your "new" plugs. You will be surprized. They can get old almost as quickly as you can replace them.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.