Bad start-up.. Need help =/

[spatpho]

Hey guys'.. I'm confused with a problem that's happening to my car. I drive a 98 civic (d16y7; automatic). To start off... When the engine is cold, the car starts up fine, sounds fine and runs fine. And after driving and putting the car into park, the car will make really loud noises from the engine bay. And if I turn off the car and start it up again, the car will start but feels like the engine isn't even running.. I popped the hood open and found that the whole engine shakes like crazy. But when I gas the car (while in park), the engine all of a sudden starts running normally. :???:. Would you guys' have an idea would could be the problem? No CEL's. And I've just had an aftermarket header installed as well about 3 weeks ago. And I've juss replace the spark plugs as well. Thanks.

[bistec4]

Hey guys'.. I'm confused with a problem that's happening to my car. I drive a 98 civic (d16y7; automatic). To start off... When the engine is cold, the car starts up fine, sounds fine and runs fine. And after driving and putting the car into park, the car will make really loud noises from the engine bay. And if I turn off the car and start it up again, the car will start but feels like the engine isn't even running.. I popped the hood open and found that the whole engine shakes like crazy. But when I gas the car (while in park), the engine all of a sudden starts running normally. :???:. Would you guys' have an idea would could be the problem? No CEL's. And I've just had an aftermarket header installed as well about 3 weeks ago. And I've juss replace the spark plugs as well. Thanks.

Is your idle low when the engine is shaking?
Have you ever touched your engine mounts?