Thanks so much for the response. There have been no warning lights at all with this problem. The people at the garage told me they got nothing when they ran the computer check. I bought the vehicle used from a subaru dealer - so that's where i took it for servicing. I'm planning on going to a GMC dealership in the morning. The really unique thing about this problem is it only seems to happen during the first 15 mins of operating the van. As soon as the engine has been warmed up by either idling or bucking and shaking around a parking lot - everything seems to fix itself and the van performs well at high and low speeds. I imagine that it'll only get worse though if i don't figure it out. Any further suggestions would be appreciated.