My 98 GT has given me enough stress to last a lifetime. I finally got the new intake manifold installed since the old one was cracked inside the thermostas housing. Id say a good 1 1/2 inch crack at that. Now my car is idling real rough. I noticed on my vacuum gauge as I was idling it was fluctuating a few psi at an idle. And when I get up to speed it jerks and hesitates a little. I just put new spark plugs in though and put some injector cleaner in it. I also double checked leaks around manifold and nothing, could it still have a fouled plug? Or maybe bad plug wires?....any suggestions, ideas...please help!!