4.6 intake manifold help

[Zim28]

Hello,

I have a 97 GT with a pretty much stock 4.6. Anyway last summer my intake manifold cracked around the thermostat housing. Well I bought a new replacement manifold, one with the updated aluminum water crossover. I switched this out but the car has never ran as good as it did before. The manifold was the correct part number for the application. I have taken it off several times since but can't seem to get it running as good as it should. I changed the plugs, the PCV valve along with this manifold as well. My car now burns a little oil. In fact when ever I take this manifold off I can see where oil is coming in through the throttle body, through one of the lines in it. Though I can drive it fine, it burns more gas, idles pretty roughly (people sometimes think I have a newer cam) and has quite a bit less power.

Though I like to do my own work on my cars, I understand there are points where my knowledge ends, and I have hit it on this. I probably will have to break down and take it into a shop. I just don't want to have to pay a huge bill if it is a simple setting that I just don't know about. Thanks for any help.