Q: Cracked Manifold

I bought my car less then 6 months ago this Summer. After I had purchased it I noticed a couple of times my car making a slight jerking motion - but it didn't happen that often. We went to the same dealership inquiring about buying a truck for my Husband and I mentioned at that time that the vehicle did that a couple of times and I wanted it to be on record however, no one had ever checked my car. Now I go in for an oil change and they are telling me that the warranty on the vehicle is up because it is a manufacturers warranty and the vehicle is a 2012. They are now telling me I will need over $300 to fix this manifold. I am not happy and I feel like they should be liable to pay for this, as I believe they sold it to me that way. Would the jerking motion be from a cracked manifold? And if so what causes this? I haven't been in any accidents nor have I put many miles on this vehicle at all - I am looking for some assistance as to where I should go from here.

My car has 150000 miles.
My car has an automatic transmission.