Developed a sputter and a heating problem

I have a problem with my caravan and thought maybe you can help. A while back I had the SES light come on, and the computer at autozone read "engine misfire" I replaced all the spark plugs except 1 (I couldnt reach it), and replaced all the plugs, but the one. I also changed the distrubuter cap and rotor. This took care of the misfire light. Then my van started to die when slowing or at a stop. I changed the front oxygen sensor and this took care of all problems (for a week). I also changed the fuel filter at this time. Now, it will occasionally get a bad sputtering problem when I am driving. not everytime I drive, its spuaratic. I dont know what could be making it do this. It also developed a problem with the heater at the same time as the sputter. It will heat for only for a while, then I turn it off and back on, and it will heat some more. Any ideas? Your help would be much appreciated!

I don't know how much it will help, but could you give us the Year and Engine size and transmission type at least.

its a 96
with a 3.0
automatic transmission