Honda Civic 2004 Check engine light coming on while going up hill?

So my Honda Civic 2004 has an extremely difficult time going up hill. If I start to climb a hill and my rpm's approach anywhere near 3600-4000 rpm's my car begins to do what I describe as "stutter up the hill." My check engine light turns on instantly. In addition to this, I can start going up the hill at 70 mph; however, when the "stuttering" occurs my gas peddal feels like it almost gets stuck and won't push any further (even though it can go further down) and my speed declines drastically and quickly. My check engine light will usually go off 24 hours after I return to a flat surface and have driven on the flat surface for about one hour. I can always tell when the light goes off, because the tension in the gas pedal is relieved and my car drives much smoother. I have had the code read twice, and the mechanic said it was throwing a different code each time so he is not sure what's going on. I have had new spark plugs put in and I keep up with my oil changes if that helps.

That "mechanic" is of no use to you… Go to an auto parts store and have someone there attach the OBD code reader to your vehicle; the code reader will show the coeds of what need to be repaired/replaced; have the repairs done & go back to the parts store; they will again attach the OBD to clear the codes from the vehicle computer

Find out what the code is and it will tell you what the problem might be. Auto zone auto parts can scan the code for nothing

You need a new mechanic.
and post codes.
go back to that hill reproduce the problem(cel light on) and drive straight to a local auto parts that do free diagnosis(scan reader) I know autozone does.
do not turn the car off in between.
depending of many things, a bad map sensor can do that, same if there's a vacuum leak. Old timing belt causes the car to loose power. Even a bad TPS can cause sputtering. A clogged up fuel filter also is a possibility.
even if there's water condensate inside the fuel tank the grade of the hill(angle of approach / ) can cause the water level to increase at the point where the fuel pump will pick it up and cause the sputtering. Have the catalytic converter inspected if you can.