Car shaking at 60mph+

Whenever I drive the car, it's fairly smooth at low speed but once I get above 60, it vibrates a lot. It's not bad enough to bother me but it vibrates nonetheless and it shouldn't be this way.

The previous owner told me he replaced the entire suspension since it was getting old, put the summer tires on and changed the faulty engine chip. That's all I know.

If it can help, I have 2 check engine lights. One is for the MAF sensor and the other one is P2004, which is something about the intake manifold stuck open.

The car has some rough idle at red lights, it shakes for 2-3 sec and goes away for another min. It seems to be lacking in power as well when you first start the car and at stops/red lights, it will accelerate very slowly until it hits about 15mph where it picks up some decent acceleration. At first I thought it was a comfort thing to avoid kicking you in the seat but I think fixing both check engine will solve the issues above.

I also noticed the transmission is a bit jerky on the 2nd to 1st gear downshift, it gives a little kick before coming to a stop. I really think it's just because the car has a lot of miles but who knows.

Now for the shaking itself, it is kinda bad at 80 but it doesn't shake the steering violently, it's more a vibration felt everywhere in the car. I'd like to say it comes from the middle of the car right behind the center console, but I don't want to be misleading.