My guess is the timing is off. Either the crank to cam timing, or the cam to distributor timing.
It was sitting in a barn for a reason. Maybe the previous owner had replaced the timing belt and got it a tooth off, it ran like crap, so they just parked it.
Did it come with any warranty from the dealer? If so, I would tell them to fix the problem, because it obviously doesn't run right.
Did it run this way when you bought it? If so, why did you buy it with it running like crap? If you got a smoking deal on it, then that would be ok....but I am guessing since you bought it from a dealer, then you didn't get a smoking deal.
The dealership told me that they relaced fuel lines before they sold it to me.
Look under it, on the inside of the passengers side framerail is where the fuel lines run. Look to see if the large line looks like it is new. If it isn't, then they lied and didn't replace the fuel line.