Kip's41,
My obvious first guess would probably be your first thought too, maybe something you have already checked. This would be the firing order, which is 153624, clockwise from the front.
Other than this, it very well could be a sticking valve, or even a blown head gasket. I would verify the firing order and take a compression check. It might be worth your time to pull the valve cover and just take a look at the pushrods and general condition of things.
Stuart
