Has your engine ever been overheated or have you had problems with cooling? 250 miles is not much to have the seals go bad, unless the guides are so excessively worn that the valve stems are distorting the seals. The misfiring sounds like plug fouling which clears when you get going. I had this happen on my '89 300E just before the head gasket went. Sitting at idle in traffic, engine would start to misfire and clear up after I started moving again. But,.. I also had oil fouling on the plugs. Have you have the drain back galleys in the head checked to make sure they are clear. if they are gummed up, oil can accumulate on the top of the head and get sucked in around the intake valves.
2000 Dodge Grand Caravan SE