2004 Honda Civic

I have a 2004 Honda Civic with a 1.7L SOHC that also has a manual transmission. Earlier this year I drove it up to Montana where a bad winter storm had hit causing the outside temperature to drop to -50 degrees F. I had attempted to start the car up and after a few minutes of working the engine to where it could start, I noticed my car driving differently while I began driving it.

1. The first thing I noticed was a total lack of horsepower. The acceleration was much slower than normal, even when holding the gas pedal all the way to the ground, it still took time to gradually increase in speed. And while driving sometimes, if I try holding the gas alll the way down to gain speed, the engine begins loosing horsepower and the car starts slowing down.

2. The second thing I noticed was that the engine could not produce a speed greater than 70mph on the highway. Even when still in gear and going down hill. The best way in which I can describe this is as if the engine is hitting the rev limiter much earlier than it should. I had even tried driving down a hill as fast as I could and even had the car in 5th gear and the car wouldn't go faster than 70 unless I put the car in neutral.

3. Third, the car tends to stall after awhile of driving or after having driven at any speed of 50mph or more. Additionally after a while of driving and beginning to feel the engine lose some horsepower, if I hold the gas all the way down while in neutral, the engine rev's fluctuate from high to low and cycles like continuously instead of remaining at a high level of rpm's like an engine normally would.

4. The engine also does sound different from normal. The best way I can describe the sound is kind of like the sound a jap muffler makes. But coming from the engine compartment itself. And in between gears while shifting, it almost sounds like a rattling noise is made.

The engine check light is on and everytime I have brought the car to Honda, they say their computer reads it as the Manifold Absolute Pressure (MAP) sensor as bad and they have replaced it twice so far and seem not to be capable of figuring out the root problem because after they replace the MAP sensor, the check light comes on again and the problems are still there.

Talking to some friends, they think I may have blown a header seal but I haven't had any problems with the radiator fluid level or with the oil being any color that it shouldn't.

Any ideas on what could be wrong?