Ok, I did some diagnostics and found out cylinder #5 had almost no compression, the others were in acceptable limits. I pulled the head off and a valve was broken, I replaced the head with a used one. (For reference #5 is on the front of the engine, bank 2)
Now, the car definitly has more get up and go, but it idles terribly. There is no CEL, but I had the codes read and it came up with P1131: (from autozone's printout)
Lack of back 1 02 transitions - lean
ECM detected bank 1 02 at it's lean limit.
Misfire on bank 1
Vacuum leak affecting bank 1 only.
Fuel injector problem bank 1.
Engine mechanical condition.
One thing to consider is that there are a couple broken exhaust manifold bolts so there is a fair exhaust leak.
I really don't think it's the intake manifold gasket. I was careful putting it on and took care to torque in the proper sequence, and it's notched to sit in a specific place.
I read around and some people said it's actually a MAF problem, but I put a new one on already.
Right now I suspect that possibly the coil is bad causing a misfire (I put new plugs and wires on the car), or that the upstream O2 sensor has gone bad.
The car has 130k on it.
If anyone has any experience with a similar problem or would like to lend some advice, i'd appreciate it.