'99 misfires periodically upon acceleration.

'99 Villager misfire upon acceleration and sometime has a rough idle, but only periodically. Sometimes it runs fine, even on interstate hwy speeds. I replaced the fuel filter, and have had the O2 sensor, Knock sensor, plugs, and wires all replaced trying to fix this problem. No luck, I was driving in town this evening and it began to misfire again, mainly when accelerating. After picking the van up from the mechanics earlier in the afternoon, after having the wires and plugs replaced, my wife took the interstate and didn't have any problems. We are at our wits end and have spent about $1500 in the past two months. Words of wisdom welcomed.:screwy:

I'd have the codes read on it. I just took our 99 to Autozone, because I have a slight stumble at idle (but runs fine otherwise), and the code for the Knock Sensor came up. I'm bummed, because I know it isn't cheap to get replaced.