1996 Dodge Caravan rough running

i have a 1996 dodge grand caravan that runs rough as soon as it warms up.thougth there was water in the gas and i put gas treatment in but did not help.doesnt do it all the time some times it runs good then it starts running rough like its not getting gas or its not firing good any suggestion would be a great help . also i recently replaced the water pump and was wondering if i migth have gotten the tim ing off a notch also could engine running to hot make run rough because it seems like it starts running rough when it warms up.but it doesnt do it all the time you can be going down the hiway and it will run great then run rough like its starving for gas then clear up. where is the feul filter located and how hard is it to change thanks