how to pin point the main problem on a ford winstar lx 2001?

we have put on new manifold gaskets on upper and lower, new thermostat ,belt tension-er, new spark plugs and wires, new air filter ,and new battery ,and a new oil change. we went followed the book how to tight them all down also so it like it still not wanting to run right .please help me ?

Alice
September 2013